It doesn't sound like she's scared of the other dogs, she's just letting them be dominant. She may well be purebred. Whether she's pure or mixed she will still have a unique personality. No breed is guaranteed to give you a guard dog, but just because she's submissive to the other dogs doesn't mean she would sit and take it from a human either. Not all Staffys are going to have the personality that they've been stereotyped as having.
At the same time, I feel like I should mention that having a dog for protective purposes is a bad idea in general. If it's the feeling of protection that you're after, then a dog is great. But if someone wants to hurt your wife or break into your home, no dog is going to stop them. A close friend of mine works for an insurance company and has seen many dogs of all different breeds "dealt with", including dobermans, pitbulls, and even two 200lb English Mastiffs. A dog will only serve as a deterrent for stupid teens, that's all. The best thing would be a home security system.
One other thing to consider would be house insurance and specific laws. Many insurance companies will not insure the homes of certain breeds and along with German Shepherds pit bulls of any sort (American pit bull terrier, staffordshire terriers, bull terriers, etc.) top the list. And depending on the state you live in you could actually wind up with jail time and large fines if someone gets bitten by your dog - even if they are on your property and even if they broke into your house and, heaven forbid, harmed your wife. Did you ever hear about the case of the criminal and the skylight? A thief crawled up on a couples' roof and accidentally fell through the skylight into the kitchen. He landed on a counter and pierced his leg with a large knife. That couple had to pay for all of his insurance and hospital bills because it happened on their property.
Anyway, I'm rambling
Her personality is a submissive one, at least toward other dogs. That's not a bad thing - in a staff some submissiveness is good because it indicates you may not have to deal with aggression problems later. It has nothing to do with her breeding, it's just the way she is..