pygocentrus cariba

I am wondering what tank size 3 adult Caribas require? Is 75 enough or will I need something like a 125 when they are adults? Could I just put babies in a 75 or 125 and be done? I have heard that there is a 20 gallon rule for Caribas. If that is true, then a 75 should be fine for 3 of them but I...

Here's a quick vid of my mixed pygo shoal. Caribe's getting big fast. Hope ye like.