Corporal goes between sergeant and private.
You cannot have more than 12 in a group. However, you can write scripts to allow the player to control other groups. This has been done comprehensively by the CoC mod, which is hosted on this site (Ed Depot) as well as having their own. This gives a lot of control which is in fact unnecesssary for some purposes. There are a couple of other scripts in the Ed Depot which you could use if the control you need is less.
Think of it as a continuum: at the top end you have CoC, at the bottom end you have a radio call for air support, which is after all controlling another squad in a fashion. It's up to you where you want to fit into that continuum.