I ve taken 20mg of percocet for dental pain I'm having, can I take my ativan (lorazepam) 0.5 mg only an hour after the percocet?

Not recommended. Both of these drugs have additive affects. They both cause respiratory depression. Care must be taken when combining sedatives with narcotics and don't ever mix with alcohol. You need to cut done on the percocet, that is a fairly high dose for dental pain.
Probably. Your medication usage should be monitored by a trained professional...Preferably the prescribing doctor. There are many drug interactions to be aware of and they should be overseen by someone trained in pharmacology.
Yes. The combination of these medications can cause increased sedation. Take with caution but generally safe.
No. 20mg is a very high dose for percoct. I would vote no and ask MD prescribing Atavan. Also, treat the cause of the dental pain, not just the pain.