Do your bettas love them or hate them? Each betta is different - mine don't seem to care but when I first added them they were kinda rough with the snail - flaring and headbutting the shell
Do they make the tank slimy (seems like a stupid question but this is an actual concern)? No, they don't make the tank slimy but they do poop alot so you do need a gravel vaccume, if you don't already have one. I do suggest a filter as well. WHat does happen though, if you have alot of them in one tank - is the water will get cloudy which is caused by "something" in their digestive track ( i think its a parasite of some sort). Don't worry, it's harmless to fish.
How easy are they to keep? Pretty easy - they don't require much but they need a lid because they can crawl out of the tank. The big thing with them is their shells can sometimes start to deterioate in softer water. This can be fixed by adding some shells, or coral. I added some coral I picked up off a beach in the caribbean (after soaking it in water for a while). They also love to eat stuff like lettuce and cucumber.
They get big though. This one grew from about the size of a nickel to the size of a golfball in like 2 months.
Besides apple snails, there are some other species - rabbit snails and nerite snails that won't over run you tank like ramshorns, malaysian trumpet snails and pond snails have the potential of doing.
I'm looking for smallest bioload possible and easy care :)
you would be OK with one in a 5G but a nerite may be better since they stay about dime sized. However I am not sure if their bioload is actually smaller. I would think so since they dont get nearly as big. Or if you just got ONE pond or ramshorn so they can't reproduce - I think they are even smaller then a nerite. Not sure as I don't have them - only the big boys - apples, trapdoor snails and one lone nerite. Rabbit snails are neat too but harder to find and more $$.
The care for all species is pretty much all the same.