Slitherine

Just to be sure :

When in your army list you may take a unit with two possibilities , for exemple in the Tatar army you may have you horse archers either CV or LH , if there is nothing specific in the army list, you may have part of your BG as CV and other BG as LH ? let's say I thake 20 bases as CV and 12 bases as LH or must I take every base the same CV or LH .
I choose the tatar army exeple because there are 2 distinct possibilities on two "different" lines.

If both options are listed then generally you can pick and choose. Some armies will have things like "all light horse or all cavalry" in which case it is one or the other.

But to clarify an ambiguity in your question, each BG must consist only of one type.