You can do whatever you want for strapping, since approvals only concern the outer appearance of the armor.
That said, Velcro isn't ideal as unless you get the industrial strength it tends to seperate from the armor, as well as making it harder to dial in your fitment every time you kit up. With snaps you figure it all out once, and unless you gain/lose weight you're good to go every single time. Rivets are only required for the higher approval levels and even then do not have to be functional.