You might get a few slugs but the rocks would create good homes for the critters that eat the slugs too. I have found that the first year after adding mulch, rocks, wood, etc. I tend to see some slugs but then I start to see a lot of black ground beetles, centipedes, and garter snakes. All of those critters eat slugs or their eggs. The result has been less slugs where I add those natural features compared to areas where I have not. I'm in western WA - we have tons of slugs over here!