ok. Eventually the snail might outgrow the tank, your going to have to keep up on the water changes though. Mystery snails poo a lot lol. I wouldn't worry about him flaring. He might be upset that he has a tank mate but thats it. Give him a few days and he should calm down. Watch him closely over the next two weeks incase the snail or plants brought something into your tank.
ok everything seems good now the only thing that i was worried about was the fact that this lady at petsmart took water from other fish and put the snail in that water and i didnt know if it was clean so i didnt put the water in my tank and just put the snail in without any water extra with him cuz idk if that water was clean but i had cleaned my tank before i put the snail in