[Baltimore Bryan]

Thumbs should be kept in pairs for the most part, unless you have a lot of expeirence (in the hobby and that particular species) and time to observe the frogs to watch for stress.

Actually I think many (though not all) of the thumbnails are less aggressive than their bigger Dendrobates relatives and can make great group frogs. For example, I've never had problems with or heard of difficulties keeping vents and variabilis in groups. Now imitators seem to be an exception, from what I've read, they are more monogamous and typically a dominant pair might establish leaving a "third wheel" or extras that don't participate in breeding and may wrestle. So I think it might be better for a pair of imitators in a 20, or you could do a group of 3-4 of something like vents.
Bryan