No other fish can be housed in a 5 gallon, however shrimp and/or snails will do great. I find Nerite snails are great for eating algae and have interesting shells. They will not breed invasively like some pest snails. In terms of shrimp, I would recommend Ghost shrimp or Amano shrimp as these creatures are see-through and are therefore less likely to get eaten by the betta.
I wouldn't mind the dividing thing, It would be cool. But, I am not sure where to get a good divider for a small tank. Actually, my experience with dividers is pretty aweful. Tried it in my fifteen gallon and they never sayed in place and it honestly just looked crappy.
If I could get something good that worked and looked good, I'd totally be for it. But, one betta in there with a couple of shrimp is cool too.
I'm not sure where you could find a divider for a 5 gallon tank - I made my own.
You can buy plastic crafting mesh at a craft store and make your own, it's not too difficult. You just have to cut it to the right size and use a sealant to get it to stick to the wall- only problem is that these are usually permanent. I'm sure you can find a way to make one that's not permanent, though, just use your creativity skills.
Bresn, I think what you said is exactly it - 2.5 gallons is fine. Much better than the tiny bowls, gives them some room to swim around. But, it's still only 2.5 gallons. It's like saying a 15 sq. ft backyard is fine for a dog to run around in. Yes, it's fine. Would the dog prefer a couple acres to run around in? Absolutely. It depends on what your ideal is, and how active the fish is. I have 2 female bettas in a divided 5 gallon, and I think it's fine. I also "rescued" them from the tiny bowls my office had them in before I started working there, so it was a big upgrade for them. Right now I have a betta in a 10 gallon community tank, and she swims the full length of it very frequently. I'd feel terrible about putting her in a smaller tank now.