[Dusted Fly]

What would you recommend for a 20 long vertical tank? I'm thinking a trio of imitators?

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.

 

[Dusted Fly]

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.
Bryan

Yes, he said he was thinking about a trio of imitators and they fall under the "territorial" group of thumbs. Thank you for specifying.