Welcome to the forum, and the Frostgrave Addiction.
I've never used, or heard of anyone using, additional rules for greater demons having magical abilities.
The greater demons you summon are just bigger and meaner than their smaller cousins, and not really the spell casting variety. If it helps, it's easy to imagine that Demons who spellcast themselves have figured out ways to protect themselves from being randomly summoned amd bound to mere mortals.