Sorry about that, I quoted the relevant parts to help clarify rather than just saying read the first post. To boil it down (again), the list needs to represent a selection of units that are closely identified with the AdMech and let the player choose how he wants to use the force.
We had a lengthy back and forth about the ultramar PDF. Skitarii are a bit more complicated than simply PDF or IG regiment because they are raised by forgeworlds and defend the forgeworld itself as well as being deployed to other worlds or areas of the galaxy. The closest we have to that is the Ultramar PDF which helps maintain defense of the Ultramar realm as well as being deployed across the galaxy with the ultramarines. That's why I picked the Ultramar PDF as an example of why the skitarii could still be classified as a PDF. However, my point there was to say that the name changed to eliminate the issue entirely.
I see you posted while I was typing and I think we are having a bit of breakdown in the definition of core
There are two "core" aspects I'm talking about. The first core is the selection of units that I see as iconic to the AdMech and need to form the backbone of the current skitarii list to create the AdMech flavor.
The second "core" is the selection of units within that group forming the core selection within the current skitarii list. That is the Sagitarii or praetorians, Skitarii DC, and Minorus.
What you are talking about right now is expansion beyond the current list, and I agree with you that core units to those lists would depend on their them and style. Core units from those lists will probably include some units from the initial skitarii list as well as other units selected from the complete list of admech units I'm trying to create up in the first post.