hello lucas, there are other points on the psp where you can solder but there quite hard to solder to. that i'm aware of you dont solder the mobo, rather the d-pad pcb, 'X [ ] O /\' pbc, analog stick and the home bar. what you could do for the rumble is try and hook up a wire to the pos and nev points on the speakers and wire that through to the motors on the game pad. you may need to change the motors as they may not be power by the psp. there quite big and may need alot of power to run them.
if you check out the psp section there is a topic about a controller hooked up to a Phatty. be a good place to start for info. unfortunatly there is no tut for the Slim (hasn't been done yet, well.... that i'm know of anyway, lol). hope you pull it off!