I would do a 24V PSU, except that i can't power my Mega directly from it. The 12V30A PSU is the only one i have around, and ya, it's total overkill.
I don't need the full 1.5A, I'm attaching a relatively light object to the motor, so i'm running them a bit underpowered (shooting for 0.5A, that's why i set the VRef potentiometer at 0.4V). The a4988 hasn't been heating up too much since it's not supplying it's max potential, but i do have a heatsink on it for precautionary reasons.
I guess my question about the arduino 5V pin, was how many a4988's can safely be powered from a single 5V pin? Not sure if i was correct about 20mA being the max current draw for that pin.