Rules for different units

I have a Battle Group of with 10x5 Halberdiers (full command). I am thinking to replace the front two rows with Greatswords 

But how exactly does this play out in combat, particularly in regard to the horde bonus? 

Example: As the first combat round begins, their arrangement is



GS = Greatsword
CM = Character Model
Hb = Halberd
St = Standard Bearer
Ch = Champion
Mu = Musician

Do I understand correctly that I since I have three full rows, I use all the attacks available in those three rows: From both characters, 20x Greatswords, 7x halberds, 1x Champion halberd attack (not two)

And that as the fight progresses, first, my Great Swords die off

Until there are none left to fill the front ranks?

BONUS questions:
1.) Where in the BRB is this explored?
2.) I suppose it wouldn't make sense to bring two standards or two musicians, right? But could you bring a second champion?