'broke-ass' can be a noun but I hear it used more as an adjective, e.g. 'broke-ass bitch' (lol this is the most natural example I can think of) I don't think I've heard stone-broke before (I'm English so maybe it's American), I've just googled it and it basically means the same thing, you would say someone is 'stone broke' and it's colloquial but not vulgar like 'broke-ass' is.

Not quite the same ... "broke-ass" is a noun, a person without money. "stone-broke" is an adjective, describing the state of being without any money.