Of my 4 chickens in our urban flock, just one likes slugs. The others prefer my garden plantings!
Like the others, I only let my 4 chickens into my veggie garden area between plantings. Right now I'm throwing all their coop litter into that area to fertilize it for next year.
They have been good about weeding the flower gardens and the weeeds that come up the sidewalks cracks, though they trample lots of plants. My day lilies, ground geraniums, and lavender got flattened from chickens walking through and on top of the plants. I got my chickens in late May (still no eggs, GAH!), which was after most of my flowers had come up. I'm not sure how the flowers will fare with chickens next spring. DBF and I are talking about fencing off an area of the yard - which is already small - to have it be a chicken-free zone.
Most of my plantings are mature plants, so the chickens can't hurt them much after the plants have finished their spring sprouting (I'm hoping). The girls tned to walk under most of my plants.
The dustbathing does prevent weeds from taking hold.