I was somewhat bored so I went through all the promotions in
Assets\XML\Units\CIV4PromotionInfos.xml, which I think is all of them, apart from a few bannor specific ones.
I sorted them by how many unitcombats they are available to, on the assumption that promotions available to many unitcombats should probably be available to commanders as well.
Available to 11 unitcombats:
Available to 10 unitcombats:
Available to 9 unitcombats:
Available to 8 unitcombats:
Available to 7 unitcombats:
Available to 6 unitcombats:
Available to 5 unitcombats:
Available to 4 unitcombats:
Available to 3 unitcombats:
Available to 2 unitcombats:
Available to 1 unitcombats:
It seems like a good heuristic, they should probably get all promotions available to 8 to 10 unitcombats; many available to 5 to 7; and very few available to 1 to 4, only Mobility II that I noticed.
Incidentally, I noticed level 2 and 3 of Force, Dimensional and Creation are only available to arcane units. Is that intentional?