Do you mean pets scale with player bonuses only that is no pet bonuses at all?
If so that a stupid idea as it either results in pets:
- Being a must have. Why? Each pet has it own HP pool and it defences will scale with player defences = defensive gear gives hero with pet way more survivability than a guy with out one. Remember you not giving up any of your heroes own survivability to buff your pets. One the damage side of things each pet is fire and forget DOT that does damage while your hero also does damage. <- No reason not to have one here either. Also if your hero is not attacking your wasting damage etc means everyone will want attack skills on player character.
- The only way the above wont be true is if pets are so weak they are effectively if not completely useless and you might as well not use them at all. <- Pets are dead.
You say it can be balanced the only way is to use pet only bonuses. This way you can use pet bonuses to make gear etc that boost pets massively but do next to nothing for players heroes. But then you end up with the same place we started so might as well leave the two completely separate.
As for Hybrids they are equally possible under both systems.