I've had several FTT chicks and most survived to adulthood with little intervention. Most had strangenesses, like being aloof from the flock and not really aware of danger. The friendliest chicken I ever had was a FTT chicken who only wanted to be picked up and given scratches. As an adult...
If she's been "so much smaller" than her broodermates since she was a day or two old, it is almost certainly not a FTT chick. I'd bet she's some kind of bantam which got mixed into the PBE bin.