Nope and Nope
Never really need a capacitor, its a band-aid at best.
and despite esg constantly saying no without giving a reason, here is why i recommend a cap. you want your subs to take energy from the capacitor when the subs have a hard peak. instead of running straight from your battery which is a strain on your alternator, you pull energy from a "storage device" near your amp. its a buffer between your amp and car battery. its better helps the subs demand for power. and instead of people playing around in this thread and just negating what each other say, check out crutchfield.com, they have everything from wiring diagrams to specifics on amps. they also have many articles on why you should have a capacitor regardless your set up. it will tell you everything you need to know. apparently past experience on the same car as yours as well as having a job with a car audio shop is not enough for me to recommend things without getting told im wrong or stupid. like i said these are my recommendations. you dont have to listen to them nor do people have to tell me im wrong. just trying to help you out o/p.
2000 v6 mustang RED: Totaled
2002 Mustang GT Convertible SILVER:
MGW short throw shifter, 4.10 gears, Bama Performance tuner, Airaid CAI, Bassani off-raod x-pipe, Borla Stinger Catback, Athracite FR500's (275 front, 315 rear), Bilstein shocks/struts, H&R lowering springs, Maximum Motorsport cc plates, Mach 1 chin spoiler, shorty antenna,