This appears to be a good idea to me. I would rewrite it as:
Gentle Casting
You add one specific target individual per your Mastery Score to the mastered spell, which does not suffer Warping by its power (ArM5 p.168). You cannot change these individuals later, but after increasing your Mastery Score you can add further ones.
- It is a house rule of course.
- It is safely isolated from the ArM5 rules, and can be easily written down and communicated.
- Players of the house ruled saga can still easily opt against it and so keep their magi compatible with other sagas.
- The number of target individuals exempt from Warping by power of a specific spell is essentially limited by practice sessions with the spell and books on Spell Mastery for it.
- The available numbers of exempt characters look feasible.
- One can still integrate new characters with existing procedures of the covenant by having the main caster of an important spell take some time to increase her Mastery Score.
Cheers