Not sure, what are you hoping to achieve with snails in your filter? Anything bigger than a baby apple snail isn't going to have much room to crawl around in an AquaClear.
If you want something to eat trapped food particles you'd probably be better off with Malaysian burrowing trumpet snails or those tiny pond snail sized ramshorn snails. I've had both living in my canister filter.
If you're thinking of doing it to help 'clean' your tank, they're not going to be of any use. They're not going to reduce the amount of organic waste in your system. Feed less, gravel vac and water change.
I have hundreds of tiny snails living in my FX5. I barely have ever seen one in the display. I guess they came in as a hitch hiker on some plants and took up in the filter. I dont actively try to get them out of the filter, but when I clean the filter, I rinse it all completely, and they still manage to come back.