if your going to run huge orchestrations, you have to divide each section, a bass patch, cello, violins violas etc and keep them completely separate, each individual part separated by its instrumentation and a separate track in your DAW sounds like a lot of work yes but if you are going big you can't cut corners. You have to separate everything to keep them from blurring together. I usually bus this into a master track to separate from the rest of the band (which i also bus other things in the entire mix)
keep eq to a minimum, i actually highly recommend bringing up your highs a fair amount so that you can really hear the fuzz of the strings, this will bring up your attack. upper mids almost seem to make the tone muddy (even more so than bass) Use only enough reverb so that you have a diffused relase that is long but is light. set the release of the actual string patch to about one second and leave the reverb to about 8 seconds, roll off most of the bass in the reverb and then bring the wet signal into the dry mix until you can just begin to hear the reverb, as soon as you hear it, that's enough, anymore is overkill.
and in order to make sure everything fits, you have to mix each individual part to the entire mix (guitars and drums etc.) Judge the final mix and make your orchestral adjustments there.
Thing is that with heavy guitars and drums or more overall saturation, the clarity of the orchestra (as well as the other instrumentation will loose clarity, just the nature of the beast).
Other than that it take experience, you have to play around and see what works, my main hints are to separate everything and keep post processing to a minimum, no compression, small eq, small reverb.