I have an Epiphone Special Les Paul Junior, and I've been thinking about replacing the one humbucker that came on it with a P-90. Any reasons why this is a bad idea? Any recommendations about what I should do or what product I should buy? Any advice about installing it?
There are no real gotchas in replacing a humbucker with a single coil, except where you have a phase switch:
A humbucker often has 4 (or 3) output wires so you can connect the two pickups in series or parallel - usually selected with a switch. A single coil just has 2, which you could connect to a point after the switch - doesn't require much electronic knowledge.
You will get better treble response from a single coil - From Gibson's own marketing "the P-90 offers a stellar combination of high output and biting treble response" but you will have a lower resistance to interference, and possibly lower gain.
If the treble is what you need, I'd say go for it. If you don't like it you can always change it back.