If when you say Dragon Puffer you mean Palembang Puffers then yes, i used to have a breeding pair till i sold them to a shop...
Aggression Levels: Depends on the individual, but being lurkers they don't move much or get too aggressive unless some one gets into their space, and being piscivores they will potentially eat any tank mate that gets near the business end of the puffer
As for housing with other puffers, i got lucky with mine coz i started off with a male first, then a few months later got a female by fluke when a store was trying to get rid of its stock... so i can say yes they can be housed together, but A) the tank will need to be large enough to break up the lines of sight, and B) have enough floor space for them to avoid each other if need be... C) they should probably be around the same size to avoid any serious damage done to the other...
My two used to get into small fights randomly, mainly from the male attacking the female... nothing too serious was ever done damage wise... just little bite marks on the body, but never any open sores...
Anyway, as i said, these puffers are TRUE LURKERS... so they will basically never move.. my female used to come up to the top when food was available, but the male would literally stay in one spot and not move even for food... or at least very little before retreating back to his little spot...
As for size... i have to be honest, the largest one i've personally ever seen in a store "looked" to be at least 13" - 15" i know some sites say they don't get that big but this store had two very large ones which really surprised me... just wish i had my camera on that day so i could take a photo...
