Caudopuncts are rock spawners, but the fry use the shells as a club house, I’ve seen the adults use shells when juveniles if no rock work in tank, but after a while the start living under/near the sponge filter. So add both options.
If you have punks and multies in the same tank, you will see disputes in regards to shell ownership. You can get away with less shells for punks, 2-3 for multies. But still adult punks stay closer to the rocks.
All said and done, this is from my experience with WC, not sure if tank bred fish differ in domestic living choices.
