Sixes ends up being the most efficient way to frag and make progress in a map, if you could add a second demo or medic then you could get away with 5 (but then it ends up being something silly like pocket Medics on Scouts with a Roaming Demo).
One thing to note: Highlander only ends up existing because it's also a natural fit (one of everything? Sure!), but nobody likes to herd cats and that's what keeps it relegated to UGC.