Yep, black saddlebacks are nasty! My female(2.5 in.) always beat the crap outta the smaller male(less than 2 in.) and killed my royal gramma. this happened in a 90 gal tank! The female was so dominant that the male would wait until she's done feeding before he takes a bite.
I had to do a major re-aquascaping before adding my 3rd royal gramma, so far they seem to get along fine.