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.
Feel free to message me any questions, I'll try to help the best I can!
Rummy- Mini Rex Rabbit
Diz jr- Betta
Yoshi- Crested Gecko