Exposing slide film isn't any harder than exposing other films. Very slight underexposure usually works out better than any overexposure. Just be careful of your technique; with 35mm, do some bracketing.
35mm slides are usually returned in mounts, but a decent lab can also return them as a strip so that you can mount your own. A "return unmounted" request may even result in a slightly lower price.
Most MF transparencies are probably viewed on a light table. There are some projectors available, but they tend to be a lot more expensive than 35mm machines. Finding a projector for 6 x 9 might be difficult.
Slides or transparencies can be printed by any decent professional lab. Most of them now transfer the data to disk and print from that instead of making direct Cibachromes. Most labs can also produce internegatives from transparencies.