Can you mix methergine (methylergonovine) and percoset?

Yes. Methergine (methylergonovine) is used to cause the uterus to contract to decrease bleeding during a postpartum hemorrhage. Percocet is a medicine that some use to treat pain after a c-section. They are often used together.
Yes. Yes you can. There is no interaction between them.
Yes. You should be fine. Make sure you drink lots of water and eat as well if possible.