Whenever I start my 94 RX7 I hear a squeaking from under the hood, I am guessing it is the serpentine belt. Does this mean it needs replacement or just adjustment.

 

until you isolate the noise you guess is as good as what anyone else is going to tell you.

 

It could simply be that one is simply loose or worn. But before tightening or replacing, post a picture of your engine bay showing the belts.

 

Could be loose. That's the first thing I would check.

 

No pictures from the OP yet. But installation of under-drive pulleys and removal of the air-pump results in very little belt contact with the water-pump pulley. This often causes slipping and noise. The tendency is to over-tighten the belt and that can cause other problems....premature belt wear and worst-case...accelerated front bearing wear on the engine (see below)