The important thing with followers/cohorts/companions/etc is to make sure they A) don't overshadow any of the players and B) don't allow the "controlling" player to dominate the game (by being over-powered, unstoppable, what have you). Since these "break" the action economy, they have to have reduced odds of succeeding to put them in-line with other stunts/powers.
The way I'm handling it in my game is a two-tier option:
For one refresh they get a companion. This is an animal, spirit, or otherwise less-able companion than a human being. As such, it cannot use basic equipment such as armor or guns. This companion has a quality rating (QR) equal to 2-less than the main character's applicable skill (Rapport, Contacts, Presence, however they flavor that they "recruited" this helper). The companion has one skill at that quality rating, two at QR-1 and three at QR-2. The companion has 2 boxes on each stress track by default and can take no consequences. The player can allocate refresh to purchase stunts/powers for the companion from their main character's pool if they wish.
For an additional refresh the companion is humanoid (or otherwise capable of using basic equipment) and gets an extra skill at QR.
So far, it's working fine. It almost seems like it's not good enough, though, so I'm considering buffing it. Better to underguess and make it better than overguess and have to nerf someone, IMO.
[Edit:] Ah, I see it's very similar to what you were proposing, Sanc.