1. Set the enable videofx focus hotkey option on the viewer, and use the hotkey as assigned on the Character Set (default ALT+Mouse1). Sometimes people don't like the double click you may need to do here, so they set on the Window Layout, the Focus Follows Mouse option, and then set the VideoFX Hotkey to Mouse1.

2. No idea what you mean here. The viewers should be using the same dimensions as the vfx source, or they should be using the appropriate ratio for the dimensions. i.e. if your source is 1920x1080, then the viewer should be 1920x1080, or something smaller/bigger, but of the same aspect ratio. I use this site regularly to save me the math http://andrew.hedges.name/experiments/aspect_ratio/.

3. No. Some people have found once you get a large number of vfx sources/viewers that they get low FPS, but not everyone. I use over 30 viewers with approximately 50 sources in my setup. I depends on which client is the active one as to which sources are currently active. At one point I accidentally ran about 50 viewers and created around 130 sources, and it still ran with no issue.