Saltwater fish question?

I have a 33 gallon aquarium, about 3 months old. I am a new to saltwater aquariums. I have lots of experience with freshwater tanks and fish but I know they aren't the same, so my question is about stocking. My tank parameters are ammonia0, nitrite 0, nitrate 3-5, pH 8.3 and SG 1.023. I have about 40 lbs of live rock, filters, heater, power head and skimmer. Now the inhabitants I have right now are 2 clown fish, 1 fire fish, 1 zebra hermit and 1 blue legged hermit. I was considering getting 2 more fish that come out more often than the fire fish. My clowns are aways out and that's what I like but I was wondering if I can mix 2 different species of damselfish. A humbug damsel and a yellow tail damsel. I have read up on them and i now that damsefish are aggressive, that's why I want to get them at the same time and they are my very last fish. Would it be ok to mix those 2 together? I have lots of caves and such for them to call their own territory. what do you think?