New addition advice

Hey people, I recently lost my sailfin tang which was added last. I have a blue tang and a mimic, the sailfin was of equal size to the others(about 3.5inches) but after about a month he seemed too scared to feed and took to hiding alll day. Not sure what caused this, a zebramosa should hold it's own against my other two tangs shouldn't it?
Now I want to replace my sailfin with a yellow tang, as far as I heard you should add these last so should a yellow of similar size be alright?
Was also thinkin of returning the mimic and changing him for a different tang, would adding a yellow and another at the same time eliminate my problems?
Lots of questions I know but what would go well with with a blue and a yellow?

 

I have a blue, yellow, and sailfin together... you could try another sailfin.

 

You may not like this response but I say leave well enough alone. The Sail Fin may be getting picked on at night and the turf wars be sorting themselves out. Yellow Tangs are good/difficult and adding or exchanging will cause stress and possible ich outbreaks or other problems. You have an established order now, give it a chance. You may want to add a veg clip so the others concentrate on it and it would be a diversion at feeding time. The Sail may nip at it periodically also. I have experienced that some Tangs (other fish also) are sensitve to Alk and PH swings/changes. I had a Yellow who told me by his Red marks on his head he was agitated and it was usually one of those parameters.