Increasing the number of taxis might decrease the number of cars on the street, especially in cities where public transport is not a viable option. Allowing more taxis certainly would decrease the demand for parking in already-congested central areas.

Definitely, although it's not clear that ride sharing will increase traffic for sure. Cabs and Ridesharing are essentially car multiplexing. Absent an on-demand ride, most people would drive into the city, and then park there.