Mixing workflow question, whats a parallel channel

I think you are mixing (excuse the pun) 2 concepts, which share a similar setup but are referred to as different things:

EFFECTS SEND
If you want to add the same effect with the same setting (e.g. the same reverb applied to all guitars to give the impression they were all recorded in the same room) you would add a new BUS and put your required fx in the fx bin with the desired settings - e.g. if you wanted a "guitar room" reverb then you could set up a room type reverb

The important thing at this point is to remember to set the fx to 100% WET - i.e. You don't want any dry signal on this bus

You then insert SENDS onto the tracks you want this type of fx - e.g. In this example you could insert a SEND on your guitar bus to the REVERB bus and adjust the send knob to vary how much of the guitar signal goes to the REVERB bus

You will then hear the dry guitar bus and REVERB bus at the same time (i.e. In parallel) so it will sound like the guitars have REVERB

You may want to apply different amounts of reverb to different guitars, so you can insert the SEND on any track and still send it to the REVERB bus to vary the amount of reverb on each track - but the advantage is that the reverb type and settings will be the same (giving your mix a more coherent feel) and you are also using just one instance of an effect so it is CPU friendly

I typically have a number of FX busses in my projects - lots of different reverbs (big, plate, ambience), a more general ambience reverb, a short delay (tempo sync'd) and a long delay (tempo sync'd) - these all then feed a master fx bus before going to the master bus

PARRALLEL COMPRESSION (sometimes referred to as "New York Compression")

This has a similar setup as an fx bus but you put a compressor on the track will quite extreme settings so it "slams" or "crushes" the signal being fed

The idea is to use a SEND to send some signal to this bus so that it is heavily compressed and gives a "big" sound - you then blend this with the original track to regain some of the transients and dynamics

I would say parrallel compression is most often used on drums (and is often referred to as a "drum crush" bus) to give that big sound but retaining some dynamics - it should work well for metal!!!

Just beware of falling in to the trap of "louder sounds better" as the ear is easily fooled

You're doing great. You're jumping in and using the software without waiting for every little answer. You'll answer some of your own questions and increasingly relevant questions will come to the top getting you going even faster.
Just remember to make backups of important projects in preparation for any oops steps that will happen and things will go fine.
I notice you have no computer questions so crashes don't sound like a problem. That's a great thing SPLAT is normally very solid when set up with a good PC.

Play on!