Oh boy, I'm sorry to say, I see some serious problems here. A 5 gallon tank is IMO way too small for 3 damsels. It's not surprising aggression reared its head. I have to ask, why, if the LFS said one of the damsels would kill the other two, did you buy three anyway? UGFs and hang on filters are fine for fish only tanks, but they're not appropriate for tanks housing nitrate sensitive inverts and corals. They're great at removing ammonia and nitrite, but tend to turn into nitrate "factories", resulting in very high levels of nitrate.
As for the 29, you should be able to add another fish or two, ut I wouldn't add the damsel. They're very agressive by nature, and will likely harass your clowns. If you're going to turn the 5 gal into a fuge, just return the damsel to the LFS. I should warn you that chocolate chip stars are not reef safe and will eat a number of different types of coral. I also should say that the BTA should also go back to the LFS or be given to a local hobbieat with the necessary setup to keep it successfully. While BTAs are amongst the hardiest of anemones, they need considerably more light than your providing. Either T5 bulbs with individual reflectors, or metal halides. Some people have been somewhat successful keeping them under PCs, but in those cases, they need a good amount of supplemental feeding which will likely foul such a small water volume. Anemones typically need the stability of a mature tank (6 months to a year) with pristine water conditions. Not to mention the fact that BTAs get quite large fairly rapidly. Like enough to fill your 29 .
I don't mean to be harsh, or say these things to discourage you, but in order to save you lots of future frustration and your critters a miserable existence. The good news is, you have found a great community of hobbyists who are friendly and more than willing to answer questions and point you in the right direction.
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