The reason for which guppies are risky, is that they are schooling fish, they reproduce very fast, and they are very colorful that brings out aggresiveness in the betta, thus the risk. Wild guppies are less dangerous because they are duller, but then again not worth the risk.
Also not any ground feeder is good to be a tankmate for a betta, some need extra tank space, like a Pleco, which can grow HUGE.
I DO NOT recommend platies because they have the same issue as the guppy, and they can be nippy. They could nip at your betta's fins and that isn't a good thing :/
For more information check here :D
I just found the site and it pretty much sums up all I think about some of the tankmates.