1.) I used an AC20
2.) You can do up to three, but it will depend on their personalities. Just try to break up as many lines of sight as possible and give each puffer a little place they can seclude themselves and call home.
3.) Oto cat's do really well and many people have had success with shrimp. I think otos will always work but with shrimp your mileage may vary. The other option may be some really fast swimming fish like danios or other type of minnow. Puffers are slow swimmers so if they can be evaded they might work. I wouldn't try anything with long fins though.
4.) Enjoy them, feed them well, and give them lots of attention. Keep the tank really interesting because puffers are very intelligent and curious creatures and if they get bored it could create aggression. Rearranging the tank every now and then is also a good thing. Do not feed them mystery trumpet snails. The shells are very hard and can break their beaks. I fed mine bloodworms, tubifex, and blackworms. They can easily be trained to feed by hand or by tweezers or tongs.