Yes Betta's will generally ignore these guys. He will act as though it's not there most likely. As for adding them at the same time... sure snails in a ten gallon won't have much of a bioload and neither should your Betta.
Bettas aren't aggressive towards snails... geez haha. That'd be swordtails and mollies.
Trust me, I've kept a lot of bettas and snails, they don't hate on each other. Your betta might follow it around and peck at it, because your betta has never seen a big golfball moving around THEIR tank... :\
Don't worry, they end up being the best of friends haha.