Kwin can work with composite and without.
It depends from availability Composite X extension.

In Óomposite mode new windows reparented to Redirected windows.

Is it possible to have _few_ windows that are not Redirected to Composite, and all other that are redirected to composite?

If I forced do 'unredirect' for some window (in X server), than I can't see it at all.

I need simultaneously few windows that are redirected to composite and in the same moment other windows that are not redirected to composite.

But it is necessary to manage all of them by kwin.

Is there method to switch some window from displaying via composite?

The main reason to do this is security aspect.

I have few windows, that are secure. And I want to protect from making screenshot of them.
Any program can ask for Pixmap for redirected window.
I can protect X protocol via SeLinux, I mean CopyArea, GetImage …