The thing about snails is that a lot of them are good at multiplying, and they poop a lot! So be careful if you don't want to overload the system. A nerite or mystery snail in a 5 gallon with your betta would likely be fine!
Note sure what other fish you could include in 5 gallon with a Betta. An African Dwarf frog would work, but you'd have to look up their care. (Good forum here: http://www.bettafish.com/showthread....can+dwarf+frog
The issue with the Betta eating the food before the frog can get it can be avoided if you net the frog for each feeding, keeping the frog/net in the water, and dangling his food in front of him in there. Worked well for my frog.
AQ salt is only recommended to have on hand as lots of owners use it for conservative treatment to various betta problems. It shouldn't be added to the water unless there is a problem with your betta's health and the recommended action is to add a little.