Hey I'm from Michigan too :) I'm a Grand Rapidian
Don't feel bad about your betta not eating yet. Mine wouldn't eat for almost a week, except for some bloodworms, and now he is a big piggie.
I think a 2.5 is just the right size for just a betta, nothing more. Don't be too worried though, bettas don't mind being alone. I too wanted something more than just one betta and got a 10 gallon w/out a lid for about 13 dollars at petsmart. I don't fill it up all the way so the betta doesn't jump out.
Here are (some) choices for tankmates with my experience, but many others will work.
Mystery Snails: Not the most entertaining, but they sure are cute. My betta did not like them personally. He tried to eat their feelers once and the snail got very scared and a week later. That was my case, but I've heard very good things about having snails too. Just be careful not to get a snail infestation.
Pros: Don't need a ton of space. Pretty to look at, variety of colors, adorable when they come out of their shell. Easy to feed.
Cons: The feelers caught the attention of my betta and he tried to take a bite out of them. When they die they also smell like the worse thing you have ever encountered in your life. They also make a lot of waste.
African Dwarf Frogs: They're cute too, I actually haven't had these myself, but someone I know does. They like to hide and if you have a hiding spot you may never ever see them again. They are also pretty much blind and feel for their food. So if you don't put their food right infront of them they'll starve.
Pros: Cute, swim up for air. Don't need too much space. Males can sing occasionally. Sounds like a buzzing.
Cons: Likes to hide, a lot. More difficult to feed. Can't be kept in a tall tank or they tire before they get to the surface for air. Can get caught on rocks and drown easily.
Corydoras: These are my current tank makes. I recommend them the most because they are so entertaining and are a great contrast to Bettas. However, they need a lot of room because they are so active. But it's fun to see them fly around the tank. My Bettas doesn't bother them, at first he did chase them but they are very fast. They also prefer sand, but I find gravel is fine as long as it's not big or sharp.
Pros: Entertaining. Easy to feed. Comes in a variety of patterns and colors. They swim up for air.
Cons: Need to be bought in groups of 3 or more. They like to school. They need at least 10gallons. More active at night.
When getting a tank mate, make sure to have a back up plan. When bringing home the tankmate, do a water change, move decorates, put the new tankmate in first, then the betta to help the betta be less aggressive to your new friend.