Actually your right, the trap door / living bearing snails don't lay eggs. They give birth to already hatched snails. I've never had any trouble with them over running my pond. The food source will control their population. It seems no matter how many there are in a pond or aquarium, they do a good job. Only two problems with these large snails in an aquarium. If they are under fed, (not enough algee, or algee waffers as a suppliment), they will die. When they die in an aquarium they will foul the water real fast. If they die in a pond, things seems to ballance out well, and narure takes it's course.
The lake here in Tacoma that I originally collected the trap door snails from seemed to have quite a large population, because the lake is always stagnent and heavy with green algee. The snails were always close the waters edge. After last years chemically controlled algee kill, all of the wild life is gone. No sign of life at all this spring.
I would stay away from the smaller egg laying snails, because they are too hard to control, and I think they eat to many of the aquadic plants.