The cost would be quite high at a commercial printer to only print 1 or 2 copies.
When printing to your printers at work, if you get the best results with RGB, use RGB. The controllers on the printers might never have been properly set up for CMYK. Profiles can be set up for files that contain RGB, CMYK, Spot, or Grayscale colors and as such, you printer might have a very decent RGB profile set up but the CMYK profile might be slightly "off". Or that's the way someone wanted it...
As for your printer at home, I print on my Canon MP560 (at home) using card stock all the time. Works fine as long as you use the rear tray. The heavier paper doesn't like using the main paper tray (it has to bend more to get from the tray under the printer vrs the rear tray).
Never tried to duplex the card stock, but I would imagine if you did it manually (send one side to the printer, then flip the paper manually and put it back in the tray and send the second side) it would work reasonably well.