I wouldn't put any other top water fish in a tank with a betta, so no Gouramis, killies, hatchets, or top water livebearers. Also, I'd make sure the betta went in the tank last. I am cycling (using a pair of guppies) a betta tank right now (5.5 gallon, heavily planted). It's going to get a small shoal of small cories (5 or so C. pygmeaus, hasbrosus, or hastatus), and three ottos before I even consider putting a betta in. Course, given that it's been a month and the algae is just about nil, I may skip the ottos and go with some RCS (if I can lay hands on them - and they may turn out to be expensive feeder shrimp).
The idea is to make any tankmates for the betta part of the pre-existing environment and near the bottom, so that they betta will look at them as just another part of the substrate.
Now, whether or not that will work is an open question, but that seems to be the uniform advice on what to do if you're not sure if a fish will be aggressive or not.