In the DBA 3.0 rules for BBDBA it says the the C-in-C has to be the element type permitted in the list for the general, but subordinate generals can be any permitted troop type (excepting SCh and similar).
If I was to field double Later Carthaginian army, for example, along with an ally. I could take 6 Sp and make one a subordinate general. I could then have up to 4 Cv (only one of which would be the C-in-C).
That is straightforward, but what about a Syracusan army, where generals can be Cv or Sp. Can I take 12 Sp, one of which is the subordinate general and 4 Cv, one of which is the C-in-C? To do this I'd be taking option one on the list as 2 x Cv, but using only one as a general. This is the same logic as the Later Carthaginian example, except there is a choice involved.