1.Where are you flying from to get to Phila?
2.Checking baggage or just carry on?
3.When are you making this trip?
We fly through Phila most of the time on our way to SXM if we can't fly through Charlotte.
We try to have minimum of an hour and we start in Harrisburg (MDT) or BWI but we vacation in Dec/Jan so weather is always a factor.
We, too use USAir and I would try to get more time to catch the SXM flight if you can.
More critical is your return flight. If that's through PHL you will definitely need lots more time.