I will not fill this reply with all my related experience with premnas biaculeatus (4 1/2" tip to tail more or less) and aptaisia and perlscale butterfly. Currently the score is maroon clown 3, BTA 0. Aptaisia 2 PSB 0.
I don't know the specifics of the butterfly(s). They both seemed fine until all the aptaisia were gone, then they ended up on the bottom. I saw one eat during the general feeding, but the clown (and to a lesser degree the yellow tang) seemed to harass it.
I'd like to try a heartier anemone (heartier than a bta?) when I get my nitrates down, but I think it would need to be at least 8" across.
I wonder if maybe I have reached the limitations of a 75 gallon tank.
Tony