Yes, but the connector must have the nominal system impedance to avoid causing reflections. You may get away with using 50 Ohm BNCs in a 75 Ohm system with cables that are short compared to the wavelength of the signal, but longer cables will be more successful with the correct impedance connectors fitted. With the kind of flat finger-type connector as used with HDMI, USB etc. it is very difficult to achieve an accurate impedance for each wire.
Yes, but balanced will go further as it can accept more degradation of the signal, but is more difficult to connect without reflection.