A little confusion
Mark, since you're here...
I'm a bit unclear on what your intent was with Group Cleave and Group Great Cleave. The description of Group Great Cleave lays out clearly who can do it, but never quite says what 'it' is
As I'm reading it, Group Cleave appears to give the members of the unit to, when they disable an opponent and are elligible to Cleave, to take a 5 foot step instead of an extra attack. I'm not sure if that's right, though, as you describe them 'clearing a path for others to occupy'.
I like the feel of the group combat feats as a whole; those two just seemed like they might need rewording for clarity. At least for me
Additionally, you might consider combining Defensive Advance and Solid Retreat into a single feat that doesn't depend upon the direction the group is moving. I suggest this mainly because even an elite regiment probably has, at best, 4th level fighters or so as its private troopers. (At least, the way the demographics seem to be written in the DMG). With every member of the unit being required to have the feat, and with the modal level of the members being low, there's a sharp limit on the number of these feats that will probably be present in any one unit.
Nice work! I'm looking forward to seeing where you go with it.
--N