damsels are a typically agressive fish but yet at the same time not depending on age. When damsels are juveniles they are similar to a clownfish and will make an anemone its home, however, as they get older they get to be a bit braver and start becoming more agressive. While some species will be less agressive...they are still agressive. Like blue said green chromis are fairly peaceful however i have found them to be one of the harder fish to keep alive. Once they are established in a tank they are fine but getting them to that point can be a bit tricky so if you do plan to take that route don't be surprised if you lose a couple, they stress real easy.