dwarf puffers, a freshwater puffer, can be kept in smaller tanks. i've kept two in a 5 gal, wouldn't push that though. 3-4 in a 10 gal will work provided you have plenty of plants and caves so they can set up their territories. dwarf puffers, though only an inch long, can be quite nippy. they should be kept in a species tank. some peopel have successfully kept certain algae eaters with them, but not community fish.
now, the puffer you have pictured there looks more like a Green Spotted Puffer, which a brackish water puffer, and gets 3-6 inches so would need a much bigger tank.
be sure you know what type of puffer it is before you buy one, what type of water it needs (freshwater or brackish water, and if its bw,make sure you know how to go about making the water brackish, and the added expense of the salt....), how big it gets, etc. do you research. puffers are NOT a beginners fish. start reading the pinned topics in teh oddball section. then read through some of the threads. DO YOUR HOMEWORK.
most puffers will only eat moving food or food that is not at the surface. this of course can vary by puffer, but thats a norm. all puffers need snails to help keep their beaks (teeth) in check. you can get these snails for free quite often from your lfs, ask around. not the big apple or mystery snails, no the little nuisance snails that come in with live plants and drive most fishkeepers batty. depending on the puffer type, other food could include live ghost shrimps, live or frozen brine shrimp (thoguh this should not be a constant single food source), live or frozen tubifex worms (keep in mind, live ones can sometime carry nasties adn not always a safe choice, i lost a puffer to a bad batch of worms once), frozen mussell, etc. most puffers won't touch flake or freeze-dried foods. my dwarfs will only eat whats moving, so i have to feed the frozen stuff, thawed, by dumping it over an airstone so it "moves".