I know Aqueon minibows come in 5g. Also standard, glass tanks with rims come in 5g. I would recommend a standard rimmed tank. The customization options are larger than with a tank you would get in a kit.
I personally would recommend cherry shrimp or amano shrimp over ghost shrimp. Ghost shrimp are very active but with their protruding eyes, humped back, and long legs, they just gave me the willies. I liked it when they kept more out of sight. RCS are cute especially when they swim around in midwater. Haven't had amanos but may be a better choice with a betta since they are larger in size.
Whether you need something underneath the tank. That's your judgment call based on what material the tank is, what the bottom is like, what material the surface you're putting it on is, etc... I wouldn't worry about it unless it's glass on glass (glass tank with no rim on glass table).
Oh about snails. Nerites are pretty and come in lots of colors. But they are harder to find and more expensive. You may want to first test out the living-together thing with another snail that is easier to find/replace/don't care about as much.
Last edited by jemminnifener; 10-18-2012 at 05:33 PM.
Reason: More stuffs