3speed

I've got a crappy old Sears tandem from the 70s(I think) or 80s. It originally came with a single speed rear wheel. I crammed a 5sp wheel in there last summer and put a claw derailer hanger and crap derailer on it and it shifted OK. If I cram a wider 7sp MTB wheel in there so that I can also put a decent derailer and shifter from the parts bin, will that mess up the dropout alignment enough to screw up the shifting? Or does the alignment have to be off further than that to mess it up?

Also, do you think a regular aluminum MTB rim will be OK for light/leisurely riding on the rear of a tandem with the rear rider being ~150lbs and the front rider not much more? I'd think it should be OK, but thought I'd ask while I've got you thinking about this situation anyway.