Places like www.importfan.com let's you do that. I'm sure that other places that sells bodykits do that as well, but they're probably reluctant because they don't want to break up the set.

As for the "height" of the kits, that'll be difficult to say because there are so many different kit manufacturers out there and they probably have their own design of the kit (ie a buddy club kit from one shop could be completely different from another buddy club kit from another shop).

Mix and matching kits is a slight gamble because although you do get the best design of part of the kit, as a whole it might not look that great. But basically it all just comes down to what you like. It's your car, and you'll be the one driving it. If you like the way that it looks, then that's all that matters.