lily99,
You've gotten a lot of good advice so far! I wouldn't worry about tolumnia being "difficult." They were one of the first orchids I ever grew, and I've never had much trouble with them. I used to joke that I was too ignorant to know they were supposed to be "hard!"
I think they come packed in moss to keep them moist during shipment, and because moss doesn't fall out as easily as other media. Personally, I would never leave a tolumnia in moss. They like to get doused, then dry completely within an hour or so. I would replace the moss with expanded clay aggregate (also called LECA or PrimeAgra -- basically little terracotta balls). If you soak the clay aggregate overnight before repotting, and run water through the plant every other day or so, it holds a good amount of moisture to maintain a local humidity around the roots while still allowing for lots of air. Be prepared for a lot of air roots, too! These guys throw them everywhere! (If you are keeping the plant in a non-greenhouse environment, you can run water through it every single day. But I've found the clay aggregate holds too much moisture way inside if I do that in a 50%+ humidity environment.)
I don't have a greenhouse, but I have a large wardian case in my basement to keep up humidity and run HID lights. When I "quarantine" new orchids, I hang them in my bathroom for the reasons mentioned above -- lots of great humidity. If I had good light from a window in the bathroom, I'd leave some there full-time. I have a friend who grows a tolumnia I gave him immediately over his fish tank, where it gets the constant humidity thrown off by the circulating water.
I think putting it with your cattleyas is fine, light-wise. Tolumnia can take really hard light, once they're used to it. Watch the leaves. If they get very dark green with a bronze touch, you're perfect. If they start to get too red, move them into slightly more shade.
Good luck! They are great plants -- you'll love the blooms!