I am an absolute beginner, but I would have made him pay two (or 3 if the Bd were irregular) PIPs for this as well.
1) The Bd(F) is
both a front corner of the group
and moving less than [its] maximum distance (P27 d 2nd bullet).
2) P28 final paragraph only says that 'A group move cannot exceed the maximum move distance of its slowest element.' (Not that you don't have to pay the penalty if you mix troop types as described in the example given.)
[As an off-beat example, I remember reading in my local paper of a Rememberance day parade. The powers-that-be hired a Light Infantry band to lead the march. When it struck up the march it raced off ahead and ended up 15 minutes in front of the remainder of the parade. They said that this was their standard march rate, and that only if they had been placed in the body of the other marchers could they have gone slower.]
3) They can still legitimately take their own skirmishers(behind them) or rear rank support, because this would not cause
both the group's
front corners to move less than maximum distance.
You are after all trying to get three separate troop types to march to the same tune
Steve (but this is only my take on it)