u need a cap if your pushing over 1000 watts. as well as a new alternator. and peak power is just as helpful as rms simply for hard hitting songs. if your amp has a lower peak rating than the subs you wont get the full potential from the subs. if the subs suck too much energy they will shut off. and if your running a cheap amp u can blow the amp and subs.
Nope and Nope
Never really need a capacitor, its a band-aid at best.