Saved 2 GSPs

they grow to 6 inches give or take so for the 2 of them (assuming they get along) i would say a 55g should suffice. as they get older they need more and more salt in there water. they are a high end brackish species that can live in full seawater as well. feed hard-shelled crustaceans as the bulk of the diet. they wont eat flake food. freeze dried krill works as well as cocktail shrimp, clams, etc

You want to get them into low end brackish soon- will help with their health. Also due to their messy eating creating a big bioload for 2 fish, you should look at housing them in a 30g. I had a lot of water quality issues with 1 in a 10g. As far as food, mine eat brine shrimp, bloodworms, mussel, squid, shrimp (all frozen-some from the supermarket), live brine shrimp, and live snails-the most important part of their diet. For more info on them go to:
http://www.thepufferforum.com/forum/ug.php/v/PufferPedia/Brackish/T_Nigroviridis/

I hope you put them into a cycled tank, otherwise they really aren't much better off then they were in Walmart. I refuse to even walk into their store. "Saving" fish only tells the store that they are good sellers, so next time they buy even more to mistreat & kill. Better off supporting a Ma & Pop store who keeps their puffers healthy & happy, preparing the proper sized cycled tank to take them home to.
An Introduction to Green Spotted Puffers

Food should be meaty foods. Frozen and then thawed blood worms, mysis shrimp, people shrimp, clams, snails, mussels, etc. all work well. Stray from feeding these guys brine shrimp as the staple of their diet - it's not very nutritious at all. Key to keeping these guys healthy and happy is to vary their diet. Feed once a day, six times a week (skipping one day for fasting). I assume you know about the puffer teeth - in order to keep them worn down, feed them shelled foods (snails, clams, mussels, live food such as ghost shrimp, etc.)

In regards to setting up the best home possible for these guys is a heavily decorated tank - give them lots of places to hide, lots of breaks in sight, etc. I use aragonite sand which helps to buffer the pH of the water near 8.0. Some like to use crushed coral as well - that's totally up to you. Because these guys are such messy eaters, over-filtration is essential - as well as 50% weekly water changes.

Pool sand should be okay. As far as MTS, I would avoid them at all costs. They reproduce one to a million and will take over your tank in a week. Furthermore, MTS snails should be avoided as food to puffers for their shells are very thick and there have been stories of puffers who have broken their beaks on the shells. If you're looking for snails to churn the sand, you can acclimate Nertite Snails. Not only can they adapt to brackish, they will do a wondrous job of cleaning your tank. As far as snail as food......I normally feed my puffers pond snails (as big as their eye). I usually get those for free from Petsmart/Petco. They usually will give these out for free since they're consider "pests".

As far as the filters, I would wait until someone else chimes in for I really do not know much about canister filters.