GSPs and Ceylon Puffers aren't particularly sociable, though like many people I've seen twos and threes working fine in fairly big (55-100 gallon) tanks.
The problem is that you can't predict whether yours will get along. I suspect it's the males that are most territorial because they're the ones that guard the eggs, but since you can't sex GSPs or Ceylon Puffers, this is rather unhelpful! What it does mean is that some tolerate other puffers, while others are outright nasty towards them.
So, the best you can do is try things out. I'd budget more than 55 gallons to be honest, but regardless, I'd take the same approach as a cichlid keeper. Take all the puffers out of the home aquarium, move the rocks about in the tank, switch the lights off, reintroduce the fish 30-60 minutes later, and leave the lights off for the rest of the day. With luck, they'll think they're in a new bit of the sea, and they'll treat any unfamiliar puffers as part of the scenery. On top of that, none will be territory owners yet (that's why you move the rocks about) and they'll start from a level playing field settling themselves in.
The only risk here is the bigger Ceylon will out-compete the two smaller GSPs for territorial space. So possibly my advice above will be terrible, and you'd be better of leaving the two smaller fish in place, and add the bigger one, hoping that he'll respect the territory-owners already there and carve out his own space somewhere else in the tank.
In fact you might split the difference: take them all out, but put the two smaller fish back 20-30 minutes before adding the bigger puffer.
Cheers, Neale
