After researching here for a few weeks, I chose rasboras and started with six due to the recommendations I received regarding their schooling nature. One died, so I have five harlequin rasboras in with my betta in a ten gallon at present. We have had no problems to speak of.
However, if I had it to do over again, I might just do the betta alone, or divide the ten for two bettas. My particular betta likes open swimming spaces at mid to high level in the tank...exactly where the rasboras like to be. So there's traffic sometimes. At one point about three weeks into it, I really thought it was stressing the betta out too much, and I put the rasboras on craigslist but got no takers. However, things seemed to smooth out after that, so I didn't pursue it further.
The rasboras eat like madmen starving, so I've found it's best to cup my betta for feeding and let him just float in the open cup in the tank for the 2 or 3 minutes it takes to feed. (He's not a jumper.) Then I can control precisely the amount of food he is getting. He is not an aggressive sort, so they will beat him to the food otherwise. The cupping is easier than it sounds and has really helped me catch him for water changes, etc, and makes it less stressful for him since he's used to it.
In addition to searching threads here, I would recommend aquaadvisor, where you can input the fish you're considering and see if you're at a healthy percentage of stock.
My daughter enjoys the extra activity in the tank, and sometimes I think the betta might, too...and then I think he'd be just even more thrilled to have the whole place to himself. When the rasboras kick the bucket, I won't replace them, even though we've had a good run.
What is tough is that I think it will depend upon your betta's personality...ours is a laid-back sort. From the descriptions of other bettas that I've seen on here, they might not tolerate the rasboras at all. But, in the rasboras' honor, they are totally peaceful and never a nip or a squabble provoke. From the recommendations I received here, I placed them in the tank first, then added the betta a few days later.
I wanted to add a little more movement to the tank for my daughter's sake, and I thought the betta might not get as bored. However, she focuses only on the betta, and he really would be just as happy (or happier) by himself as he makes his own fun.
Home of Buzz Betta 3 and the Rockin' Rasboras!