The French fascist regiment, similarly, was generally attached to various divisional formations. Could these tiny regimental units be represented as support units?
With close to 4000 men they would be the biggest SUs, but not the smallest on map units. May still be good to get rid of on map clutter. But would also take away some on map units that are useful as garrisons which they could not do (alone) as SUs.
If they were considered an organic part of units like 100th division then they would have to have the same class of ToE as other light divisions, or a special ToE class of their own. If I understand rightly they did not have the same range of equipment as other wehrmacht regiments.
Raises the question why are merge rules so rigid, more restrictive than documented historic practices. I guess we have to subordinate historic accuracy to the limitations of the game's architecture (one of the recurring frustrations we accept as a trade off for making such an enormous undertaking "playable"). If you want to play the side with more flexible command options, I recommend learning/choosing the Soviets.