There is no hard, fast answer to your question. It all depends on what you are trying to do. Some say that you should have 1 or 2 snails per gallon of water, or for a 125 gal tank up to 250 snails. Others would keep a lot less, and still others none at all.
Using 10 snails is such a large tank would be ok as far as the snails go, but if you want to have a reasonable clean up crew, then it would be best to add a lot more. I'd tend to start with about 50 snails of mixed species and see how the tanks does. More can always be added later.
There are several species that go by the name of blue trigger. However, it's safe to say that any trigger will eat your snails. they are very good at hunting them, and have the jaws to crush smaller ones. I think your going to need to make a choice between snails and the trigger.