I use quite of bit of MIDI when it comes to writing, mainly exported from Guitar Pro. My question is; is it possible to expand the amount of MIDI instruments I can use? I'm assuming I'll have to do this after I export to (or even edit it in) my DAW, but is there anything I can download to add more or even create new MIDI instruments?
The amount of MIDI you can use in any given DAW is virtually unlimited, it's when you start loading up the software instruments that play back the MIDI information that things can get hairy (as in taking a toll on your system resources). To add more MIDI instruments you'd have to acquire more software instrument plugins or a hardware synth, that kind of thing. I would look into something like SampleTank because you can load up one instance and run multiple instruments from it (up to 16 channels I think).
yeah, MIDI is really just instructions that tell a synth (in your case probably the microsoft wavetable general MIDI synth) there are tons of different virtual instruments out there. and you can use as many at once as your DAW can handle (this varies largely from instrument to instrument as some are very resource light and some can max out the RAM/CPU on all but the best computers)