Well, some fish will work fine...Cories and Otos are the best bet. It is my personal opinion that in the long term they might actually appreciate the company, or at least be glad not to be totally isolated. That's not to say that keeping Bettas alone is bad...they are perfectly happy living alone as long as you fuss over them plenty.

Just you can't keep them with anything that may be even tempted to nip them, or may be tempting for the Betta to nip. Not a lot of fish that fit there. :P The only problem I see with small catfish is that Bettas are generally kept in 5 or maybe 10 gallon tanks...a group of Cories may overstock that small of a tank, and it is hard to keep the nitrates down in a tank of that size, which will not make Otos too happy. So in some cases it may be best to leave the Betta alone.