Hard to say, they might be fine until 6" or so, or they might kill each other at 3".I just got 4 one of them is about an inch and a half the other 3 are about an inch.There in a 75 gallon I'm assuming there's 3 boys and a girl only because 3 are a yellowish colour and one is black how long can I keep them in a 75 b4 they try an kill each other?the big one is already nipping at the others but lots of hiding places tho.
What's your goal? Keep them all, or keep your favorite? If you want to keep one, I'd wait until about 3", and keep the dominant male. If you want to keep them all, I'd spring for a 200+ gallon tank within about 5-6 months.