For carbon fiber, you absolutely need the top cap to keep the preload. It's probably never a bad idea even on steel.

I just want to check my understanding. You want a quill stem on a threadless fork. The only advantage that a quill offers is that you can adjust the height. I suppose you could turn it sideways by only loosening one bolt it single bolt threadless stems exist.

I have quills on a couple of my bikes. No complaints. I can't really see going to the effort if the fork is already threadless.