I can't technically confirm it's Marian Reforms that triggers the new bodyguards but I'm sure it is. I modded Capua to be a huge city and every time I play, the second turn sees all my generals having upgraded bodyguards, regardless of faction.

What I wonders, however, is...can we hurry the Marian Reforms? Is that "upgrade_bodyguard" trigger in exports_buildings file the Marian trigger?