to maximise the growth of the remainder, try get them a 55G for themselves, do 20% water changes every other day and feed small amounts 5 times a day, keep as much flow on water as possible because then there will be more oxygen in water giving the fish more energy then they will grow faster..
festae are very slow growing as im sure you have found out.
there will always be a few who dont grow very fast if your keeping them in one tank as there will be a few dominant and then a few who dont get much food.
in my 80G growout ive got, 6 F1 N.grammodes 3 of which are about 2.5" and the other three about 1" due to the dominant others