I have an Atlantic Orange Tail Damsel in my shallow 125 gallon along with a three stripe damsel and a bluehead wrasse, Niger Trigger, and a Snowflake Eel. The atlantic Orangetail is the most aggressive fish I have ever seen, but does not bother the three stripe at all. It occasionally goes after the wrasse, especially when it was first introduced. The wrasse seems to have worn it down with its persistence. It goes face to face with the trigger sometimes, with neither one backing down, but neither one attacking each other either. I have had some other small damsels in the tank at times, and although it did show some aggressiveness, it wasn't excessive unless they went into the heart of its territory.I had a Harlequin Baslet in there also and they fought a lot at first, but both learned to respect each other and coexisted nicely after that.
I've also kept a yellowtail damsel in a 10 gallon with a Sebae clown with no problems, and there is currently one in my 55 gal with 2 dwarf angels and a tomato clown with no problems.
I have read posts before of people keeping a lot of Damsels together with no problems. I believe because if there is a lot of them it reduces the territoriality behavior because it takes too much energy to police a territory.