I think you're talking about the yellow tail damsels... if so... from what I've read and seen in others tanks, they can hold their own and may be the aggressors, even towards bigger fish. (small fish syndrome)
Yes, there are a few types of coral and rock epoxies that are reef safe....check with your LFS
Your 29G is not a big enough tank to keep a bunch of fish. I would say that if they both make it through the cycle and you plan on keeping them both, you may only have room for one more.... such as a clown, or something small. Lots of LR
and a good skimmer will reduce your bioload to afford this but the general rule of thumb is 1 inch of fish per 5 gallons.
If you decide to add more LR
, it MUST be CURED. Otherwise, you will start another nitrogen cycle and the ammonia spike will most likely, kill your fish.
you can get some astrea, cerith and nassarius snails to add to your clean up crew and some more hermit crabs.