Interesting! Haven't heard that one before.
They will get eaten.
Isopods move fast enough to be spotted but aren't fast enough not to get eaten. It sounds like a very expensive way to give your axolotl a snack.
Even more, their carapace might not be very suitable for axolotl stomachs - though I don't have experience with that.
If you could get a large enough population that they can out-breed the axolotls appetite, it could be interesting (and probably be beneficial to your ecosystem), but considering the risks I would be very hesitant.
Do you have any experience with the aquatic isopods? I must admit I'm not familiar with freshwater aquarium isopods!