I'm picking up a 69 subframe from a TC member that has a C5 brake setup on it. If I take the calipers off, can i put a 15" rally on it to just move it around?

If it uses a stock Camaro spindle, then you probably can, assuming the brakes at 13" or so. Just measure the inside of your rally wheel..if it's less than 13", then it wont work.

If it's using a C5 knuckle, then it won't work at all. It's too tall.

if you have the caliper off the rotor should fall right off with it and then your rallys should fit, unless like Matt said it has a C5 spindle in which case i dont know...