None of these fish species are appropriate for a 2.5 gallon tank. Bettas are really the only fish that can live in such a relatively small tank. If anything, putting another fish in there is going to stress your betta out more. Snails have a lot of waste, so I would recommend not putting one in a small tank like that. You'd have to do some serious water changes. If you really feel the need to put another creature in the tank, go for ghost shrimp, but understand that in all likelihood, they are going to end up as betta munchies. Remember, bettas are by nature very solitary. Think carefully about whether putting another fish in there is something your betta wants or something you want. Decide responsibly
If you want to make your betta more comfortable, get some decorations or plants so he has lots of places to hide. Try live plants if you want! Anubias, java fern or moss, anacharis and wisteria work really well in a 2.5 gal and can get by without dirt and with low light.
Good luck with your betta!
I find switching around decor helps keep my bettas interested. Even in a ten gallon with a solitary betta I wasn't able to keep him from attacking everything else (including 5 tetras, 12 ghost shrimp, a pleco, several snails; all at different times. ALRIGHT! I get it. You want the tank to yourself lol).
Every other water change or so I switch the decor of their tanks, which keeps them entertained and gives them something to really investigate. I've heard of people with success with a ping pong ball, air stones (bubbles!!), new plants, and the like. Variety are a betta's best friend :D
In that size, I'd be hesitant to get more than a shrimp, and they like to have like, 5 to school in. Good luck! :D