Question: Fish

What is the best type of feeder fish for a fire belly newt? I have 7 gallons of water that can be utilized.

Assuming by the way you worded it, theres some sort of land area in the tank. H. orientalis(Assuming you're talking about Chinese Firebelly Newts) don't use much land, and only require a piece of floating cork bark if anything, mine never used it, so I just removed it. The minimum volume of water is usually considered to me 10 gallons, to keep the water parameters stable, and stuff.
Anyway, H. orientalis don't usually live in ponds with fish in the wild and will sometimes refuse to enter bodies of water with fish present. Even if they enter the water with fish present, I doubt they would even be able to catch any. Plus basically all petstore newts are WC and are usually in bad condition and stressed and sometimes even die within a few days of purchase, and finding CB H. orientalis could be a challenge.
Some newts are better at catching fish, like paddletail newts, but it is nearly impossible to get any CB paddletails. And feeder fish aren't probably the best thing to feed them anyway, a good staple are chopped nightcrawler/earthworm.