If I were to introduce only 1 assassin snail, and it were a female, is there a chance it would be carrying fertilized eggs? or, is fertilization done late in the game?
It's possible for it to lay eggs alone but as far as my experience, the females started laying eggs only after a series of mating with the males. How would you know the female has been mated already? I've yet to hear from someone their only assassin snail laid eggs. All the cases I heard involved several series of copulations.
The only way that would happen is if she had mated previously at the sellers holding station. It is also real difficult to tell the sex of an assassin and can only be done by observing a mated pair. There are a few tell tell signs like size and stripes bit this is more an educated guess than anything else.
I kept dozens in a 10 gallon tank for months. After I moved the last ones out at least a dozen mid-size MTS were climbing the glass, and there were more on the bottom. They will control but not eliminate MTS, though the ramshorns were a total loss a long time back.
I think if you introduce one Assassin you should be fine. I have 9 of them (Assassins) in a ten gallon with live plants and various pond snails. They eat about a snail of their size daily, and don't bother the snails after they have eaten.