Questions about underdrive pulleys

Hi, I have a 1990 Plymouth Sundance with the 2.5L TBI engine. Current mods include:
-MSD 6AL ignition with MSD Blaster 2 coil and Bosch Platinum plugs (.035")
-Custom ram air intake system with K&N drop-in
-Custom 2"OD exhaust with 18" glasspack (soon to be changed)

All of that was done by the previous owner, except the intake setup. I just bought a Dynomax Thrush Turbo muffler and I will be installing that this weekend with a (mostly) 2 1/4"ID custom exhaust system. I'm thinking about going without a catalytic converter as well (I know its not street legal, but I don't have emissions for a while, and by then the car will be gone).

Anyways, I'm thinking about an underdrive pulley for my car as one of the final performance upgrades. Its not the quickest car in the world, obviously only having 100HP/135LB-FT stock with a 3spd auto trans, but its pretty decent for being setback by those two things. The pulley is going to cost me like $100. Now my questions..

Are underdrive pulleys interchangeable between cars? Not all cars, but would I have a chance of being able to use the same pulley in my next car? Would I be able to save some money and buy one of the ones on eBay for a different car? Would it be safe to get an underdrive pulley even though I have a stereo installed (Alpine head unit to Jensen JXP720 amp powering two Vega 12" subs)? I was considering increasing my idle speed from 750-800RPM to about 1000RPM if I got a pulley anyways, just to make sure that my charging system stays decent enough.

Don't waster the money on that car.
seriously.
Underdrives are good for 5-10hp on a car like yours.
if you ditch the cat, hold on to it and put it back on when you go to sell your car. noone will buy a car with no cat.