compatible Pumilios

I purchased 4 d. pumilios about 7 months ago. I've kept them seperate, putting each one in its own 5.5 gallon tank (as recomended by p. nabhors on his website). I really only meant to keep them this way for a short while but time went by and I had a couple bad experiences mixing other dart frogs (in bigger tanks) which led me to just keep them this way. I kept them this way also because I have read so much information which tells how aggressive they are and I did not want to lose them. I am planning on putting them in a 58 gallon aqaurium within the next few weeks. However, two days ago I broke one of the tanks and had to put two of them together. I came back to check on them three times (They are at my parents house and I am in the process of moving out). Two of the times they were hanging out together and the other time I visited one was up on the glass and I could see his vocal sac moving. Not as expanded as I have seen in some pictures where it looks like a huge bubble but about half that. I heard no sound coming from the frog. I was just wondering if this could be a pair? The "calling" frog is considerably smaller than the other frog even though at the time of purchase they were the same size. There were no signs of aggression such as hiding or being pushed into the corner. Is it possible that this is a pair or is it not enough time to tell. I seperated them again today when I replaced the tank. If they are a pair I guess I would like to keep them seperate although a lot of people suggest keeping them in groups for breeding. Any thoughts or comments would be appreciated. Thanks

[Dane]

Any and all pumilio that I have kept (Bastis, MC, Chiriqui, BriBri) have had audible calls, usually much louder than anything else in my frogroom, so if it was indeed a calling male, you should have been able to hear it. As for temperment, there are more aggressive and docile species, but it really comes down to the individual animals. So far I've had good luck with groups of 3 (1.2), but I've also had some male frogs that I just couldn't pair up, even with a willing female because of how nasty they were. If you're planning on keeping them all together, I would do so, but keep a close eye on them for a few weeks. Any problems should be evident by then.

[Frogtofall]

All 4 are the same morph too right?