Another foresight comment but... if you had bought some marbled crayfish they produce quite a few young that start out life at 1/4" and in less than a month can be over 1". Takes from 4-6 months for the crayfish to start producing eggs and then another month before they hatch so not a solution by any means. Do you think the small freshwater gammarus would do? I have read numerous posts about people talking about how prolific they are and some even called them invasive in an aquarium without predators!
If what I read is true you start off with a few dozen and in 4 weeks you have swarms of 1/4" prey for your rays. How many shrimps do you feed on a weekly basis anyway?
That said marbled crayfish might still be a good investment as they can grow into something your rays can still eat as adults? Just my .001