Yes, couchettes are differ a bit in hardness, but not everyone can sleep on them (I can about half of the time); in other languages they're called "lying-down cars", as opposed to "sleeping cars", where you get a proper mattress. I've never seen someone use a sleeping pad on top, but you could definitely try; I have thought of doing that too but not tried it yet. It would be easier if you get on a bit earlier in the evening, or the starting station.

I usually use a thin (cotton/silk) sleeping bag because I can't be bothered with the provided linens. No sleeping pad, and I don't think I've ever seen anyone use one. I think the padding is fine, it's certainly not unpadded. The more expensive compartments do have more comfortable beds.