I love snails, so I've been considering getting one mystery snail for my aquarium. My boyfriend thinks it's a bad idea though and told me to not get one.

It really is MY aquarium (he bought it for me, but he does help me with water changes and takes care of it when I go out of town), but I still don't like to do anything that I know will make him mad (I'd rather not pick a fight!). I've been doing a lot of Googling on this topic, but I still have some questions and have seen contradicting info. So...maybe some people here could clarify some things to see if either I can convince him or if I'll end up agreeing with him?
1. If I get one mystery snail from a store, I shouldn't have to worry about a snail outbreak, right? My understanding is that you'd need a male and a female for that to happen. Building off of that question...
2. I read that if you have a snail that lays eggs out of the water, you need to leave a gap between the top of the tank water and the top of the tank so that there is room for the eggs to be laid. If that doesn't apply, is there still a need to leave a gap of open space like that for a snail?
3. I read that you shouldn't get apple snails because they can get so huge (I have a 29 gallon tank), so people say to get mystery snails...but then I read stuff that apple snails are a type of mystery snail...? If that's true, is there a way to tell the difference?
4. What is the possibility of a mystery snail bringing a disease into my tank that will harm my fish?
I might be forgetting things, but if anybody could give me help with any of these questions, I'd appreciate it.
