The Kilcher Homestead is a 600 acre Pioneer Homestead that has been protected for perpetuity via a ConservatIon Easement. The Kilcher Family has also preserved the Historic Cabin and site as a LIving Museum.
Yes, negative pH is possible, but not in water as a solvent. The range of pH in water is 0 to 14 (to be more precise, it is -1.7 to 15.7, but for simplicity's sake, we start with the usual values.) Thus, what pH indicates is the ability of the acid to form H3O+, which can then donate a proton.
By addiing additional salt to your solution you have increased the ionic strength. The pH is formally defined as the negative log of the hydrogen ion activity. The activity is calculated by mulitplying the activity coefficient gamma times the hydrogen ion concentration. Therefore increasing salt, increased the ionic strength, decreased the activity coefficient of the proton and your measured pH is therefore less.