How much space should I have between framing studs and plumbing/electrical pipes?
Can interior "walls" be norrower than the usual 4"? can i use the 2x4 with the 4" side flush
To the ceiling to maximize height of ceiling? How much skill will I need to mud the drywall
After it is installed? Should I hire someone with experience?

The elctrical pipes can touch the studs but plumbing should stand off and be braced with a plastic hanger to prevent noises. If you tried the 2x4 on edge your wall will start to wave and have curves in it, don't do it. If you have never done drywall hanging and mudding I'd suggest a pro. Hanging isn't too tough but there are tricks to keeping the drywall edges and seams right on the studs. Proper mudding can be almost an art form. I know how to do it and have done it many times but usually sub out to a mudder. They are quicker and do a perfect job. Get 3 bids but don't necessarily take the lowest bid.