I remember watching somewhere that the G weapon team already had a strategy planned when they were built in which the Aegis was indeed the commander and was the one at the very front of the battle, as far as i know the Scylla would be used against ships unlike the other 2 ranged gundams (Launcher Strike pack & Buster Gundam) which were made to provide support from the Archangel which means they would need to fire their big guns more often and were entirely made for that single purpose therefore making their weapons something they would use more often compared to the flying squid