You're correct. The Slimline RGB LEDs can't be connected to the non-dimmable splitter. You'll need a dimmable splitter for each Slimline RGB.
The RS232 version is only needed for the connection to the ProfiLux 3 and only one Mitras Slimline or Lightbar splitter can be connected to the ProfiLux. Additional splitters must be connected to the previous splitter or an Expansion Box.