I do the bulk of my mixing on headphones, and I've learned that you've got to be careful when judging reverb when on headphones; it's easy to apply too much reverb and it becomes quickly noticeable if you listen back on speakers.
As for collaborations, I seem to be the odd one (in that I do a _lot_ of collaborations) and it's usually me asking and actively recruiting people rather than the other way around. The workshop link in the menu can be a good resource to find people to collaborate with, as you can search by skill/instrument there. As to finding a guitarist, normally you can't throw a stone without hitting at least 3 guitar players, but on OCR they seem to be in hiding or running away from me, since I haven't been that successful in finding a willing guitarist here lately... Maybe they think I smell funny