I had 5 damsels in a 20G tall tank for 6 months or so. I have a picture of it:
This is a picture of when it first started, it's the only picture I have of the whole setup.
Granted, the two fish in front grew to almost double their size during the time they were in there, and things got a little crowded, but they all made it through. My advice is to try and have as many hiding places as possible.
As far as filtration goes, I put an AquaClear 30 on that tank and it seemed to be enough. I would be surprised if the same thing wasn't enough for you as well. If that's all you can do, it's certainly a lot better than letting them deal with the ich, that's for sure.