I want 4 or 5 RBPs in a 75g. At the moment I have a 20g empty. I was already advised to get 6 semi adults or 10-15 1" ones in case of cannabalism. If I got the 1" ones would I be able to keep the in the 20g for a short while or should I go for the 75g straight away and should I buy them small or big.