I never again will do the "mixed pygo" thing for various reasons I won't go into. I found keeping all one species much better. I loved the uniform look of a single species pygo shoal. A tank full of caribe for instance is very intense.
While mixing pygos, I've only mix a shoal of cariba with a ternetzi and piraya all 6-7in. The piraya was the only one getting pick on. After a week I had to move him out to my friends 55g. I sold the piraya and ternetzi because it looked weird in a shoal of caribe.