How much resistance is too much resistance for a video switch?
All the video processing I've seen happens at high impedance, then the video is re-amplified for transmission.

For example, in my sketch below, the resistance of the switch doesn't matter because it is feeding the high-impedance video buffer amplifier.

If you search your favorite distributor for "video buffer amplifier" you will find this is a relatively common part.  You can find them with fixed gain of two for exactly this application.