Percocet and Diphenhydramine are both CNS (Central Nervous System)depressants (among other things). It can be risky to combine two CNS depressants at the same time. (they will both make you sleepy...
Yes, this would be OK. Remember that Percocet is very addictive. Take it exactly as your doctor prescribed and don't take extra. Don't take other pain medicine on top of this.