Here is a profile of harlequin rasboras:
They are a shoaling fish, so get an absolute minimum of 5, more is better. Recommended tank size is (slightly) larger than a 10 gallon, so probably 5 or 6 would be your limit if you decide to get them anyway.
Dwarf are definitely a better option if you can find them. I would get a group of 10-12 in a 10 gallon with a betta, if that's all you're going to have in there. Mosquito also could be a good choice, with 8-10 of them in there.
Rasbora males can get a bit nippy, and they're quite fast. Some breeds are worse than others with this. If you refer to the link above, you can read profiles of several other types of rasboras - just keep in mind that they are all shoaling fish, so you'll want to get at least 6 of which ever you can decide on. 6-10 in a 10 gallon with a betta would probably be your max, unless you got the dwarves.