I say it's up to the envoriment and set up. 2 RED bellies in one that can go well with breeding pair. MOST pygo can not get a lone in one tank, like the strongest survice.
My answer would be both..some piranha owner may have luck some dont.
I personally had 2 RED in a tank and it doesn't work, but I have 5 in 75gallon that would well over 6 month.
ive had 2 in a tank for over a year (not anymore) and they where fine but i beleive if its weak and the runt it will get killed if its in a shoal of 2 or 20