I love urchins, pin cushion ones, since they stay small, doesn't sting you, are immune to hermit crabs and other snail predators, don't die like snails do... so many pros, so few cons. Yes they move stuff around, like any other animal who can grab a shell or loose rock, Not really an issue if you glue it down. If you glue something to a smooth surface, it could pop off, but if you glue onto a rough surface, or something with a feature, the glue/epoxy will enter the feature and anchor it down.