do you culture anything else like worms already? slugs pretty much breed and grow in the same conditions. i seem to have a large population of slugs in my backyard, even during the colder months so i don't put much effort in breeding them since there so readily available. really i try to only tweezer feed caught slugs since i don't want them sliming up the tank, competing for food with the other beneficial bugs (pill/sowbugs,millipedes,sprintails,earthworms), and nibbling on live plants but will let them roam if not accepted. they will easily breed if given the chance.
in my tanks i have a few areas of leaf litter, fruit/veg waste, old moss, feces, and dead prey(mostly DOA waxworms and uneaten worm chunks) covered with rocks. i keep these areas moist like the rest of the tank and let nature run it's course. the slugs will breed, lay clear jelly circle eggs under the rocks and babies will hatch