I have two damsels in my tank. One (Jenna) I've had since the beginning of time. She's mean, territiorial, and would chase any new fish I introduce into exhaustion, if my tomatoe clown (Rob) would let her.
I my case I have a tomatoe clown fish that has been in the tank longer than the Jenna. He's "owns" the tank, he's the boss (he even enforces fishie bedtimes). I added 3 green chromis and a purple pseudo chromi a few weeks ago. Within 3 days, Jenna had killed off one of the green chromies. Since that day, anytime Jenna messes with one of the new fish Rob will chase her into her corner and guard the smaller fish.
If you have had your damsels for any length of time, and if they are considerably larger than the clowns you introduce, then chances are there will be problems. Then size of you tank will also affect their aggression-- smaller tanks cause bigger turf wars. One way to aleviate this problem may be to rearrange your rock prior to adding the new fish. If the damsels are more concerned with their new surroundings, then they might leave the clowns alone long enough to feel comfortable defending their own territorry. . .