I've had a couple of Golden Apple Snails for about a month.....
Are they dirty? A bit, but they clean as they go, so, IMHO, they make up for it.
Can u feed them algae wafers? No idea, I have sunk blanched cucumber and courgette for them though.
Are they harmful to your fish? Not that I know.
Do they need to be in groups? Don't think so, my 2 wander around and only bump into each other from time to time, but they seem pretty content.
You forgot - are they cool? Yes, I didn't really believe they'd be as bright as they are, and they're constantly on the move, they also cleared a bad patch of
brown algae from the back of my tank in no time.
Now all I want to do is control the other little snails that stowed away on my plants !!
Kes