Just another data point - I tried an oto in my 2.5g with one betta. They got along fine, but there wasn't enough algae to keep the oto busy, so I had to supplement with wafers, and found that the oto produced enough waste that I had to clean the tank twice as often. I do have one in my 5g with one betta, and it works well. I have 3 otos in my 10g, and about once every 10 days or so I give one of them a little vacation for a couple days in the 2.5g, then put them back with their school. I don't know how much stress this causes the otos, but they've been with me for a few months and *seem* to be fine. As soon as they go in the 2.5 they get right to work, and in a day or two I have a clean tank and a chubby oto...
Stacey