I know that some people have had shrimp to survive in their axolotl tanks.
Not much, though. Not enough to fully feed an axolotl.
Shrimp that survive in an axolotl tank typically do so by hiding out and being lucky. These shrimps don't need to be fed. The food you give your axolotl is enough nutrients to feed the shrimp.
But only a few shrimp can be lucky enough not to get eaten - barely enough to sustain a population. Certainly not enough to be more than a tiny snack every now and then.
If you want to have a population that is big enough to actually matter in terms of nutrition, the population would be too big to hide and be lucky - most of them would get eaten, and you'd need a massive population of shrimp to outbreed an axolotls appetite.
Doesn't mean it can't be done, but realise that you would have a shrimp tank with an axolotl, not an axolotl tank with shrimp. I think you'd need to start out with a couple of hundred shrimp (depending on their size a little less) - and you would need to feed these. After all, you're indirectly feeding your axolotl through them.
I think your best bet would be a separate tank for shrimp, and every time the population in that tank becomes unsustainable, you drop some of them in with your axolotl for a fun little snack. Maybe some survive by hiding in the plants - but that's an added bonus.
