They are least aggressive when compared with other damsels, but they will be the most aggressive in your tank. I started my tank with 2 yellow tails , 2 dominos , and one three stripe, and gave the the dominos and three stripe to a friend keeping the 2 yellow tails thinking they were not aggressive at all compared to the dominos and the three stripe, then I added a couple gobies, a pair of snowflake clowns, and a six line, all over a few months, and once the yellow tails figured out that they were now the boss of the tank they harrassed all the other fish except the clowns, they especially picked on the six line.
Also they are hard to get out, I've caught my clowns before and they are easy they are pretty trusting of me where the damsels go straight into the rocks anytime a net is near the tank, so if you put some damsels in be prepared to basically break down your tank to get them back out.