I have always used 9v batteries with pedals except for those without the option because when I was playing in bands, I didn't have pedals. I never needed a pedal board and just used a couple of pedals as I wanted to, when I wanted to. Now I am thinking about going to some sort of pedal power and in fact several years ago I bought some sort of pedal power unit (not Powertrain) with IIRC a wall wart and connectors for some pedals.
All my pedals say 9v but some say 150mv, some 800mv one 1300mv, but most just say 9v. Boss pedals say to only use a certain Boss power adapter, I'm sure different ones will work though.. So.. my question: Does it matter if I power a 800mv pedal with a 1000mv (1v) or 1.3v adapter AND what about using AC power and a transformer to power pedals. How does it know the pedal requirements and what if I'm sending 1.5v to a pedal that has a 15mv requirement? Is there anything I should know? Anything I shouldn't do if I have an adapter that is close but different, like the pedal that requires 800mv, the closest I have is 1v... then what about the older pedals that just say 9v?
I'm thinking of building a board and want to know.. I don't even know the questions to ask other than those I asked, but if I build a board or maybe a couple with two power supplies, are there things (power things) that I need to know?
Also. Most of my pedals are some sort of boost, OD, or Distortion and I usually run them into the input. I've never really understood the FX loop because of mostly just using pedals to goose the input. I've always sort of thought that Boost/OD/Dist pedals should go in front of the input and modulation type pedals maybe into the loop BUT last week I got an EHX MEL9 and it says that is should be used in the input, not the loop. So what are your thoughts on the order of pedals? Say I'm using a Boost, a Fuzz, Distortion, something like a Callista (tremolo/chorus), a MEL9, and a Wah? Is there a right or wrong order and are there some that should be in an FX loop instead of into the input?
 
Two questions there I guess, probably more since I don't even know what it is that I don't know.
 
Thanks for any help.
Julien

I'm using a 1spot power supply to power multiple pedals. Your power supply needs to provide as much or more power than your pedal requires. More is not a problem. If you have multiple pedals add their mv together and it needs to be less than what your power supply provides. You can think of it like a water cooler. If you want 8 oz of water the cooler needs to have 8 oz or more. If there are 32 oz of water in the cooler that is not a problem, you'll just have some left over. Check the polarities on the pedals, not all pedals are the same. Your PS should have some adapters. 
 
There are guidelines for fx ordering, but some things are a matter of preference, say fuzz before or after wah, I like fuzz before, but I like the more synth like tone that produces. In general compression first, then OD/fuzz/dist, then wah, then modulation, and that is followed by time based fx, tremolo/delay/reverb. Time based fx are better in the fx loop and modulation can go either before the preamp or in the fx loop.

You are also confusing mv - millivolts with ma - milliamps. Pedals are typically designed to run 9 volt DC power supplies. Some can use more but don't have to use anything above 9 volts. Each pedal will require a certain amount of current based on how complicated the circuit is. Drive pedals are simple and use maybe 150 ma (milliamps) of current. A delay pedal with more complex circuitry will require more current, say 500 to 1,000 ma. All of this stuff is direct current, DC. Back in the day old pedals could plug into the wall because they had transformers and power supplies to create 9 volts DC from AC line voltage. 
So the critical things to look at are the current requirements of the pedal in ma and the current output of the wall wart in ma. Typical BOSS wall warts are 250 ma and good for many things. Something like a MEL9 may require 1,500 ma. Usually when a pedal requires a special voltage or current requirement it ships with its own power supply to prevent problems.
As to the order of placement, well, that's really a trial and error thing. There's tons of info online.
Oh ya, the other thing about pedals is the polarity. Some run the center pin on the adapter as positive while others run it as negative. IF you get a magnifying lens (I need one) and look at the jack on the pedal the polarity will be marked showing the pin polarity.

 
I use the Dunlop DC Brick. I have a few pedals that can use 18 volts and this Brick has three 18V power outputs so I use the 18V for those pedals. 
There are seven 9Volt outputs for 9V pedals. I have a couple of 12V pedals but they came with their own power adapters. If all your pedals are 9V then get a power brick with just 9V outputs. Like what was mentioned in the other posts you need check each pedal for it's MA (milliamps) and get a power source that has more than your pedals equal. The DC Brick has 1,000 milliamps.
 
 
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Check out the Voodoo Lab products, particularly the ISO 5. The individual outputs are all completely isolated, so you can handle center pin both +/- AND get rid of ground loops in the process. 

      Typically if you are using the guitar amps pre-amp circuits for distortion then you would use delays and chorus in the effects loop. This way your distorted pre-amp signals get delays.(Other wise your clean delay repeats each drive the distortion which is a pretty ugly sound.)
      This is also handy if you wanted to set-up a stereo signal such that the main amps pre-amp distortion gets sent via the effects loop to a device that then splits it's effect into stereo outputs ( such as a chorus) which you would then input into a second clean signal amp.  If you're using a clean  amp and deriving distortion from pedals the it typically goes through distortion boxes first and through delays /modulation later in the chain, though modulation pedals can create a distinct and cool sound when run before distortion or fuzz circuits. You will likely choose delays and reverb at the end of the signal chain. Wha pedals are typically at the front end of the chain but can impart a unique tone after fuzz or distortion pedals.
 
As for power, probably 80 % of guitar pedals use 9 v DC power supplies. There are some pedals such as the Line 6 modules ( MM4 - DL4 ) or the Jam-Man loopers  etc that use 9 volt AC supplies and require 1 milliamp. 

I pretty much only ever use 2 or 3 BOSS pedals.
The first pedal in the line is their TU tuner, which doubles as a power supply, accommodating up to 6 (i think) other pedals.  Assuming they're either BOSS or similar power-pullers, i'd look into getting their tuner.  It's a tuner, a power supply, and a great on/off switch for the instrument.  This keeps me down to one  wall-wart.  
 
I don't know if that tuner would power other pedals with a battery.  I've never tried it, but if it works you'd have a plug-free line of pedals that hopefully wouldn't drain a battery in one set.

Thanks everyone. I think I understand everything well enough to move on. I figured I could find plenty of "how to" stuff online as far as placement but I wanted to hear what you guys think. I never thought about the negative/positive thing, that is important. The total voltage of the PS covering the total of all the pedal requirements makes sense.
I now understand what to try in the loop too! 
BTW, the MEL9 IS different. It requires 9.6v and 200mA. It is negative in the center, just looked. So, on something that takes a different voltage I would need to use it's PS and not the common one supplying 9v. I am guessing that it would run on 9v but not quite as well. It has no battery compartment, I guess the 9.6v requirement is why.
I was reading over the weekend something from a "boutique" pedal maker who said that he does not make a battery compartment in any pedal because very few people use them anymore. That makes sense too. No gigging musician would want to be constantly changing batteries.
Thanks again.
Julien
 

The Voodoo Lab pedal power unit and the T-Rex mini fuel tank (the 5 output red one) have worked well for me on two different boards for around a decade each.

Both have power outlets with individually isolated grounds which prevents hum being induced in the pedals. Though pedals can still be daisy-chained from a single output if necessary so long as the total power (amps) draw on the output doesn't exceed the specified rating. They also both have transformers, not switching power supplies which can induce whining in some pedals. Unfortunately most of the "wall wart" type supples around nowadays are switching types (including the Boss I believe) and some digital pedals or analogue pedals with clocks in them such as chorus or delays really don't like the high-speed flickering "fake dc" output from most switching supplies.

dunlop dc brick.

 
i have 2 fx pedals that draw 18v, and the dc brick just happens to have 2 18v outlets!
voila