I know it seems ugly now, but I would go with a mystery snail. Your betta isn't going to like the extra fish company in the 2.5 gal, but he won't care about the snail. And, honestly, I love my snails. I thought I'd hate them. But I love them. They're so entertaining. I can't even describe to you the level of amazement I brought myself to when I realized that I really liked my snails. And that I...actually thought they were.....CUTE!
Point is, bettas can get along with fish, but they need room for it. Plenty of it. They also have a better chance if the betta is a female, or if the other fish isn't very colorful or has short fins.
Bettas would prefer your attention over a tank mate any day.
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