it's hard to know what to do when we're not exactly sure about the cuase of fishy sickness. I usually quarantine the sick ones for three reasons.. I can treat the sick one more aggressively, lessen the risk that other fish get sick plus it's chepaer to treat a smaller volume of water and the water changes arn't as time consuming. Sometimes I will treat the main tank simultaneously but less aggressively.
But it doesn't hurt to treat the whole tank if you know what's wrong. If it is bacterial I would have thought something like furan2 would have been better. Pimafix is primarily an antifungal although the bottle does say it is for bacterial as well.. just not sure how potent the antibacterial porperties would be. I've got aguppy in QT right now with pimafix for torn tail that wasn't healing, so I'll be watching to see how helpful the pimafix is as it's the first time I've used it.
Anyway, good luck with this. I had heard that dwarf gouramis can be hard to keep alive but my two are doing fine so far, despite one almost killing himself twice!