I sometimes use an 8" f/4 Newt for video - very nice
So a 6" f/4 Newt is no problem.
Be aware though that the 1.25" 0.5x focal reducers are not for Newts. These are designed for use with convex focal planes, such as those produced by refractors SCTs and Maks, not concave focal plane produced by Newts. From what I've been able to see, there is one focal reducer for Newts, and it is available Mallincam. I don't have one, and all about them is from reading reports about these, and I am considering one.
Alex.