My pad always goes outside my MLD Superlight bivy, and I don't use a groundcloth. I use my pad (usually a short z-rest) for sitting on breaks, so it's generally pretty dirty. Everything stays cleaner with the pad outside. If it's really cold, I stick my backpack under my feet; otherwise, the bottom foot or two of my bivy touches the ground. I coated the bottom of the bivy with sil-nylon, so it's non-slip and fairly waterproof.
I don't move much in my sleep, so I almost never roll off my pad.