If you get a paddletail, any small fish thrown in should be considered feeders. They are effective predators! Any fish you add should be at least 2" long and fairly robust.
There are darters large enough to avoid newt predation, including logperch, greenside darters, and large individuals of rainbow, fantail, etc. You should be able to find these locally. Dace and smaller shiners are good tankmates. Avoid creek chub, sculpins, and madtoms; the chub gets too large and predatory, madtoms have spines that could hurt the newt, and sculpins have both problems.
Bass can eat fish nearly their own size even when young, and it is illegal to take home small bass in most states. Some sunfish, such as greens and warmouth, are big-mouthed predators. Bluegill can also be nasty and grow quickly. A young longear sunfish should be OK in the tank for a while but will eventually outgrow it. I suggest avoiding centrarchids altogether.
If you want to buy some tankmates, hillstream loaches might work. White cloud mountain minnows would like the conditions, but would probably be quickly eaten by your newt.
The newt will need a small island or two to crawl out on; a large stone or piece of driftwood will do the trick. Substrate selection is a little more important with the newt; either use sand or large (3/4" or larger) pebbles. The newt could ingest smaller pebbles and get an impaction.
The water flow from your air pump and filter will probably be fine. You can always add a powerhead later if the flow seems inadequate.
