Can you use a 6/7 Speed Derailleur on a 5 speed bicycle? I bent my derailleur beyond repair today. I'm an inexperienced cyclist.

I have a 5 speed Takara. Can I/Should I replace it with a 6/7 speed rear derailleur? I am having trouble finding a 5 speed one and the local bike shops are suggesting I just get a 6/7.

In addition, how difficult is it to install the derailleur? 

Thanks.
If I'm not mistaken, you're talking about non-indexed derailers, plus the chain width would be the same for 5 and 6/7, so there should be no problem.

Installing a derailer is a fairly simple task, in and of itself.  But you either have to break the chain or disassemble the derailer to get the chain threaded through it, and that's a little more involved.  Plus you need to adjust the derailer once installed (though adjusting a non-indexed derailer is almost trivial).

None of this is beyond the abilities of someone with some mechanical ability and a modicum of patience, but it requires thought, care, and a few tools.  And note that you'll maybe want to replace the shift cable at the same time, as the existing is likely too frayed to reuse.