Pellets are fine for a puffers diet, as long as its not exclusively fed on just pellets alone and gets a varied diet...
As for the 3 fahaka's in one tank, i guess it is just a matter of time before aggression levels build and one potentially kills the other 2, although judging by your 2nd post, it seems you plan on returning 2 of the 3, and eventually keeping just one in a 120G tank?
If thats the case, maybe an idea to get rid of the two extras now and just focus on one that you like most?