i have a 99 323, should i use premium gas or not?

Absolutely! This car requires minimum octane rating 91 AKI(US and Canada) or speaking of European octane rating 95 RON.
You can go with mid grade, but you will loose some performance and gas milage.
D.

Yes, use premium

If you want to try out regulaor or plus once just to see how the vehicle runs it shouldnt hurt the car. BMW does reccomend premeium for a reason, but trying plus, then regular once wouldnt hurt your car.