creating a template for a fixture wiht multiple RGB channels

No, it is not. SmartFade ML has only one 'colour mix' controller per device, thus multiple sets of RGB can't be controlled.
For this reason, such 'multi-cell' fixtures are specifically excluded from the personality database.

The preferred solution is to patch each cell as a separate device, as you then have all the usual controls - including a 'virtual' Intensity for each individual cell.

If this isn't possible, then you can build a User Personality to put the extra cells onto the other parameters that you find most suitable.

I was thinking just the same thing last night, thanks for clearing it up.