There are all kinds of ways to deal with familiars/pets using the game system in place, but there isn't going to be a standardized way to do it because the method you use will vary by the type of flavor you want.
Examples of different ways to handle pets/familiars:
- A stunt, using normal stunt rules with pets as the flavor ex. Wolf Companion (Survival) - You have befriended a wolf that is faithful to you and follows your orders. When your wolf companion is with you, use survival instead of weapons for attacking.
- Summoning, without binding. You then use a social conflict to convince the summoned creature to help you in the way you want.
- Summoning + Binding. Use this if you want to have control over the creature you summon. Because of the way the turn economy works, this method is extremely powerful, and the high complexity needed is justified.
- Plot Device. This is how I'd handle Sue. Harry getting the word of Kemmler and consulting Lash about it gives the GM justification for giving Harry this nifty one time use help.
And more, which I'm sure I'll think of later.