rule of thumb is that NGTs hate each other, but what about a 6" NGT n a 2.5" NGT? big dats don't care bout small dats, so would a big enough size difference between two NGT make them compatible?
The 13" and the 9" would stay out of eachothers way since there was no need to challange for territory and the little guy would just hide behind the 13"er when he managed to piss one of the indos off.. amazing to watch the interaction.. i know several people who have had little to no problems with keeping 2 or more in the same tank.. it all comes down to the individual temperament of the fish though..