Sorry to read about the bad news. It's a sad fact that many of us have been through a similar loss.
Yes, they are indeed brackish fish. Aim for a target specific gravity of between 1.005 - 1.008.
Try offering them snails and live blackworms or bloodworms if you can find them. Ghost shrimp or small fiddler crabs (sorry, cute little crabbies!) might get them eating, too. Don't be too worried for a couple of days, though, because it just may be that they're needing to settle in to their new environment and will start eating once they're more relaxed.
As for tankmates, with brackish water you don't have a vast array of choices. Google (or Bing) for brackish fish and you'll get an idea as to what's out there. However you have to remember that your F8 is a puffer, and puffers tend to kill all kinds of things. However since I have no personal experience with this species, I'll just hope that someone who does will post something in answer to your question.
Oh - is the 24 gallon tank fully cycled? I sure hope so, if you have your F8s in there.
There is an article in the PufferPedia about Figure 8 puffers. It'd be a good idea to read through it.
