Snails are terrific poop producers and need 2.5 gals. of their own. I wouldn't try one in that small of a tank.
It sounds a perfect size for your betta. Put some pretty plants in there (live or silk) and you're good to go.
My dude has taken over a 5 gal. all to himself. He bullied a snail and I had to remove it. LOL. I'm afraid to try anything else with him. I got a small clay pot and "planted" some java fern in it with some gravel. It's cute looking and my betta likes to lay in it. Plus I've got silk plants.