heres how bettas go, they are all different like people. Some are more mild and others are really aggressive. The only way to tell is to try, if the betta wont get along with other fish, then fine. Dont force them.
Some poeple claim that bettas wont like fancy guppies or fish with long fins. Thats fine, but i am staring at my betta now in my communtiy tank with fancy fish. So again it is wrong to class all bettas as aggressive or wont get along with fancy fish.
but to answer your question: i have found that bottom fish like ottos and cories are usally 100 % sucessful. But it also depends on your tank size as well. But regrading that, also sword tails and the other larger "big 4 as i call them" mollies etc... will generally be fine with bettas.