If you're primarily going to add new modules then I'd power them from the a wall wart (hopefully a few per wall wart)... there's a module for distribution of this, but it might be better to have a hole in the back of the case...
Chromagnon definitely power from a wall wart - it'll probably be a massive power hog
with the psu3s - I think they're definitely underpowered at 84hp/9u for video (& imo digital modules) - I'd probablyt want 1 every 84hp/6u - but it really depends on if you're going to use predominantly analogue modules and/or use the edicated power supply for video... so maybe a bit less than that... if you're guesstimating power consumption 10mA/hp for each of the +ve & -ve 12v rails is a good rule of thumb for video modules (remember to leave some headroom)
mixing audio and video in a single case is a good idea... utilities especially can be used for both... I made a series of instagram videos #lzxveurorack, a while back, to illustrate various non-video modules working in video...
what are you using for your vector scanning??
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities