would a regulator drain a battery?

duster 318. battery was dead the other day, tried to charge and well long story short my car killed another battery.

while i was at auto zone the fancy thing he hooked up to my battery said the regulator was bad, so i threw on another one.

my question is would a bad regulator drain it over night? being it hasnt drained it since then im sorta asking a silly question

i also have a alternator from a 87 d-150.(truck) i want a higher output alt but would that mess anything up in my car? havnt really looked at the plugs yet but im sure someone already knows on here

i also see people unhooking the alt guage in the dash. whats the advantage of that?