I'll be setting up a ten gallon and just don't want to add fish, too hard to find ones that aren't nippy. These seemed perfect, being relatively small (about 1-2" tops), aquatic, low bio load, and just really neat.
I have two in my 10 gallon with my Paarthurnax and so far so good :3 They are betta safe, but you just have to keep an eye on your betta to make sure he's not bullying the ADF. Also have to make sure your betta isn't stealing all the food.
It varies based on the fish -- some might try to bully 'em, but usually they seem to get along just fine. Just depends on how aggressive your betta is, really.