Snails can count as up to 5 gallons of waste, if you get a big one (and you should, or he might become a betta snack.) I keep a large white mystery snail in my 6.6 with my betta, but 5 gallons is really pushing it. If you're willing to change the water twice as often then I say go for it... just keep a close eye on your parameters.
Snails really just need some algae growing in the tank to be happy... I think you may be able to give them algae wafers if there is none, not 100% on this. I don't actually "feed" mine at all, he's only there because without him the algae gets a little out of control.
Congrats on cycling your tank... can't wait to see betta photos!!
