No DM ever has to follow the rules. Whenever you play any game, you don't have to follow the rules.
But the further you go from the rules, the less you are playing the game you said you were playing.
It's just another example of how changing the rules breaks other rules. By making familiars fighting companions, you make beastmaster less special.
I'd rather have them make familiars capable of channeling all spells so mages can stay back and use their familiars as spell launchers than have them be combatants. At least that's unique.
