I have been using carla with various software synths for s long time both at home and live.
I have been using midi keyboards that has been easy to change midi channel so my setup was aimed to change active synths based on midi channel.
In carla I route my keyboard or keyboards to midi filter and connect my softsynths to one of the 16 outputs. If needed I chain the synths to compressors or other plugins and route them to a physical output.
My regular setup consist of SetBfree on midi ch 1
Pianoteq on ch 2
Various U-he synths on ch 3 to 9
And a few sample libraries above that.
Some routes includes more than one synth and if needed the setup can filter midi on note number as well and by so having layers and splits..
The downside with my setup is that it is aimed at maximum 16 sounds bit that can be worked around by using program changes to expand. I could send pgm changes to Pianoteq to change presets and the same goes for other synths.
By using carla and all available midi filters there are no limitations other than the imagination.
Regard Anders Hellquist