I have found that yes, they will eat and eat until they look like they're ready to explode. They will eat all of the "blender mush" I put in there leaving very little for the hawk (although he does get some). I have found the Hawk will eat flakes and the puffers won't, so I can at least get the Hawk what he needs.
As I've said, before they start getting too big, we will be getting a bigger tank. I'm thinking like a 75 gallon. Tanks and stuff go so cheap on Ebay. This is by far the cheaper of my hobbies, but requires just as much maintenance (my signature explains it).
They are doing very, very well though. The hawk has a personality too. He will watch us, and does all kinds of cool things. The kids love both the puffers and the hawks. They're a lot of fun to watch. You can almost play with them through the glass.
My other money pit...er...hobby is a Turbo Saturn.