In case of a discussion in the troupe, you anyway have ArM5 p.160 Wizard's Communion: "Communion is a remnant of Mercurian magic, so spontaneous spells may not be cast by this means, and it does not perfectly fit into the guidelines of Hermetic theory."
Multiplying number of participating magi or raising communion level by casting a remnant of Mercurian magic mastered for Multicasting may fail on many levels: easy to adjudicate is limiting Mastery special abilities for this spell family (which includes also TtA p.75 Wizard's Vigil), and disallowing Multiple Casting, Fast Casting and others which do not smell ancient Mercurian. The HoH:S p.34 box Mercurian Spell Mastery Abilities look all pretty adequate here.