the fuse is their to protect the vehicle, NOT the amplifier. JL puts protection within their amplifiers to protect against any faults/failures within their units.
The fuse protects the rest of the car's electrical system from shorts on the amplifier power lead. With this in mind, a 100a fuse will work just as well as a 50a. Both will open with any short circuit on the power wire. 12 volts directly to ground will definitely blow a 100a fuse. In fact, if you drive the 500/1 hard enough, you can blow the 50a fuse on load if you do not use the correct type (AGU glass type, I believe). These are slow blow fuses that will allow for a brief musical peak that would normally blow the fuse. THe 500/1 is rated at 500w for 1.5-4 ohms. At 12 volts, this is roughly 40a of current. During a musical peak (bass hit, etc.), this amp could easily draw more than 50amps of current (600w). In other words, just make sure you use the correct fuse that JL recommends. If you have anything else connected to the power lead, you must use a fuse larger than 50amps too...