Anything or nothing, so long as it doesn't suck up too much moisture from the clematis. No need to plant anything at the clematis' feet, so long as they're getting plenty of moisture.

I don't plant anything at the feet of any of my clematis so I can't help you. Sorry.

I love low sedums, but I would keep them away from the clematis stems. Keeping them in check is easy. Just tug them out and poke them in the ground elsewhere. They grow well in full sun, but will also grow in part sun.