Millipedes like moist environments and soil, so the easiest way to control millipedes is to turn over the mulch or soil and allow it to dry out and cut back on watering. They eat decaying organic matter, so dead or dying plants should be removed. In general, millipedes aren't harmful unless you have 100's in a single bed, and in that case, moisture reduction is really the only thing needed. There are no pesticides that are labeled for millipedes specifically, but a generic bug and slug bait may work. As long as you are not applying containers and containers to the garden, it should not be harmful to cats.