If you are going to give it a go, give this page a read, first:
Caudata Culture Articles - Fish with Caudates
It is unlikely that your axolotl will never eat the fish. A well fed salamander might eat fewer fish. Plant cover will probably be the biggest variable in your control, though. I do sometimes keep fish with my axolotls. They do eventually get eaten, but of my two tanks, the one with thick plant cover tends to give the fish a better chance. One species that I might add to the fairly-safe species listed on the above link is Heterandria formosa. I was able to find some online, and after quarantine, they lasted longer than anything else I had put into my tanks. (They're also size and temperature appropriate.)
One other cautionary word I'd throw out there is that, if you do not normally keep fish, it's probably not the best move. They come with their own needs and dos and don'ts, just like salamanders, and it could lead to more trouble than it's worth if you're new to them. It might be interesting to set up a species-specific tank of the type of axolotl-safe fish that you're thinking about adding to your salamander tanks. That way, you get accustomed to the fish's needs, and it would be easy to quarantine the fish, or treat their illness first, if needed, before adding them to the tank. Depending on the fish you choose, the tank might even become a breeding tank, and you would have a constant supply for the inevitable day when the axolotls had eaten all of the fish you put in.
It will happen. It's just a matter of when.