I also think that certain shrimps would be perfectly fine. I would recommend Red Cherry shrimp or any of the other colors. They're very small, seem to be harmless from everything I've read. They are very hardy and reproduce very quickly and easily, so you could set up a RCS tank and transfer some to your axolotl tank, and if they get eaten then they're free.
I set up a RCS tank about a month or so. 20 gallon long (75 liters) and added 5 RCS and some plants and I think I have counted about 10, and usually there's more than you'll see. I have yet to put them in the axolotl or any of my other tanks as I'm still waiting for their population to grow more. If you wanted to give them a higher chance of survival you could add lots of plants like java moss and some densely planted areas and they should hide there easily.
Edit: Be careful with ghost shrimp. There are 2 kinds I think and 1 kind has pincers and are aggressive and they might have a go at your axolotl. I've read of them attacking betta fishes, and they go unnoticed because they're so hard to see.