I always thought my fish were eating them but of course can't be sure as I didn't stock them in any huge number either.

I think they do prefer certain kinds of grass/weeds as the ebay seller did say he had to find them in a certain type of vegetation in local bodies of water.

But the water characteristics also play a big role in their survival. I would test your water. Of course adding the limestone for cover for crayfish is good too.

Many here would warn you to put a crayfish predator in before crayfish especially if you are going to give them lots of hiding places in the limestone. Once crayfish get ahead of your predators and have places to hide it can be very tough to establish any vegetation and the predators will not catch up with your crayfish numbers.

Another order of action might be.
test water first
if it could stand more alkalinity and harness add limestone
add forage fish, scuds, shrimp, smaller forage etc
add bass (if that is your predator for crayfish)
add crayfish last