I agree with Jennewt but I have had some eat least killifish (Heterandria formosa). It might be hard to maintain healthy water parameters with only 6-7 gallons of water, bust least killifish are one of the smallest species of fish in the North America and probably don't make too much bio-waste. I still don't think I'd recommend it, but it's probably your best option if you really want to try.
Red spotted newts are notoriously fragile as pets. They seem to suffer from all kinds of problems like fungal and bacterial diseases, or even just drop dead for no visible reason. While it might not be the exact cause of death, stress will certainly not help them fight off these problems and adapt successfully to captivity. Forcing them to share a small tank with any kind of fish will stress them greatly.
There are plenty of suitable foods for them like earthworms, live Blackworms or aquatic invertibrates and insect larvae. To give them any chance of long term survival I would recommend moving them to a planted tank with at least ten gallons of water. Try and feed them a diet based on earthworms with any of the other things I mentioned for variety and they will at least have a chance of surviving.