I have watched dozens of video tutorials explaining the use of plugins on pop/rock songs.  I’m currently working on a song consisting of only orchestral instruments (EastWest Symphonic Orchestra).  What sort of plugins would be appropriate for this kind of mix?  Thanks for your input.

A good starting point would be Dimension Pro. There are several expansion paks and other goodies in it to try out. 

By plug-ins, I assume you me virtual effects, not virtual instruments.  I would suggest that if you need a lot of compression for the orchestral tracks, you're probably doing something wrong in the mix.  But I think one of the very most important things is to unify the tracks into a coherent orchestra with a single, common reverb using sends.

Work your midi CC's like crazy. Listen to real orchestras and try to get as close as you can. I send all sections to a dedicated bus and use the bus compressor on each section to even them out a bit. With EWSO you shouldn't need reverb as the samples have a ton of natural hall sound on them. While they are already panned in their orchestra seating I will pan a bit farther out to taste and use Channel Tools sometimes for a wider sound. I normally have vocals and electric bass going in the center so it helps with the mix to get a bit wider sound. 

Dmbaer, yes, I should have made that point of processing orchestral tracks would be different than processing a rock mix.  The song is more classical oriented than pop oriented.  What about the use of console or tape simulators?  How about the SSL bus compressor?  I certainly wouldn't need any delay or modulation plugins.  The one common reverb makes sense.
 
(And bayoubill, I do like Dimension Pro a lot).

One of my favorite plugins for mixing strings is the Waves TG12345 channel. It has a very sweet EQ that adds presence to strings without adding harshness. Really, other than EQ and a good reverb, you should not need to add much else to a pure orchestral arrangement. You want the wide dynamics to remain, so compression should be used very judiciously, if at all. Now, if you are mixing orchestral stuff into a pop or rock song, then you'll need more compression to get everything to sit well and be audible.
 
Regards,
Dan

Something helpful (before even starting) is the choice of instuments that are used to avoid frequency collisions. It is very easy to pile on things in the mid-range that all conflict, so using a HPF and LFP on parts to remove unnecessary content will help the mixing process. Moving parts in and out of focus is another common practice... a part that is fairly consistent but not in focus will get more compression and lowered in volume so it is still heard, but not distracting.
 
This is a good interactive chart of the frequency ranges of common instruments.
http://www.independentrecording.net/irn/resources/freqchart/main_display.htm
 
I am a bit hesitant to mention plugins specifically, since there is not "stock recipe," but will say that "less is more" does apply in many cases (both in instruments as well as plugin usage).

RobWS
What about the use of console or tape simulators?  How about the SSL bus compressor?

As we always like to say about mixing: if it sounds good it is good.  Adding some warmth courtesy of vintage fairy dust provided by emulations of classic gear could indeed make things nicer.  There are innumerable options here.  Some of my favorites come from Nomad Factory (the Analog Signature Series and the Pulstec EQ in particular).  With Nomad Factory you must be careful not to overdrive or the sound degenerates to crap.
 
As to using a bus compressor, the only caution is to not remove the dynamics of the original sound.  A real orchestra has a huge dynamic range and composers often take full advantage of that.  It would be wrong to remove that from the original recording (even if the recording was done using virtual instruments) with compression.  On the other hand, compression with an extremely light touch using a vintage-modeled plug-in could be delightful.  Same with tape simulators.  Just experiment and see what floats your own boat.

dcumpian
One of my favorite plugins for mixing strings is the Waves TG12345 channel. It has a very sweet EQ that adds presence to strings without adding harshness. Really, other than EQ and a good reverb, you should not need to add much else to a pure orchestral arrangement. You want the wide dynamics to remain, so compression should be used very judiciously, if at all. Now, if you are mixing orchestral stuff into a pop or rock song, then you'll need more compression to get everything to sit well and be audible.
 
Regards,
Dan

This. If you use compression,just tickle the mix a little. Fast attack, slow release. Tape simulator only if it's a good one like the uad studer a800. Use the best reverb you can get.  Also, if you're writing in sections it's nice to mix in the sound of a first chair. It makes a huge difference.

This forum is more focused on orchestral mock ups and arrangements. You may find really helpful
https://vi-control.net/community/