try the subtractor synth or malstrom synth. see what you can get out of them first. if you don't get what you are looking for then google for the freesound project.
ya, open up rex loops and close the sustain, release and attack, then lower the decay, try shifting the octave of the loop and messing about with the lfos, you can get in on each individual slice and change its pitch and pan too, can make for some interesting loops.
no rules... just get in there and start wrecking stuff...
if you take the careful route, you won't have fun and won't learn anything. best thing to do is just to start fucking around with everything and learning what things do... if you don't discover the sound you're looking for, you're still bound to discover other neat things and you'll find the sound you want eventually... unless you find something way cooler along the way and start pursuing that.
"...Michaelangelo is a PARTY DUDE.. *PAARTEEEEEEE!* "