Trumpet snails can tend to be nocturnal, although you will see them out in daylight as well.
Breeding should not be a problem. I have hundreds now. When they do finaly have a big outbreak, you will know... they will be EVERYWHERE. I don't worry about them though. MTS don't eat live plant material (at least nothing I have in my tank) and I have heard the same from others as well. They are great scavengers.
As to puffers- If I recall, they will eat them. So do clown loaches (although they seem to prefer common pond snails).
The only problem I can see with MTS is that they will compete with your various shrimp and other things that require calcium for shell making... but on the other hand, when they die, their shells decompose (slowly), and I am assuming this puts it "back".