Hiya I love Tiggy. Your girl sounds lovely, give her a tickle from me.
I can't say I have EVER heard of any dog "turning" if it hasn't had a litter-sounds like a ludicrous old wives' tale to me. I don't have a girl, but I have seen lots of advice regarding speying on here: it is usually recommended that a bitch shouldn't be done until after her first season (usuaklly 6 monthsish, no?) Her previous owners, with all due respect, sound a bit suspect. Do you know them? If not why are they still involved? It is sad to hear a vet promoting what seems to be a myth, as most say there is no need for a bitch to have a litter. Why bring more dogs into the world unneccesarily when already rescue centres worldwide are overrun? And especially with Staffs.
You already know, I am sure, how responsible you are by wanting to get her speyed :) I would really recommend it.
In terms of breeding, a link posted often on here, which is VERY informative, is:
http://www.dogstuff.info/to_breed_or_not.html
Terrifying! lol. I am sure others here know far more than me. x