Concentration gradients across membranes with different ions
I try to answer your two questions briefly:


Ion gradients are dependent on charge, but there exist independent transport mechanisms, extending diffusion. Trans-membrane transport proteins can specifically move only one sort of ions. Also symport or antiport exists, that can depend on gradients of other (e.g. non-charged molecules). In addition, the mentioned inorganic ions can form chemical bonding to organic ions for example, which neutralizes the electrical charge.
Beside an electrical potential, also chemical (concentration) potential can act as force on ios. In addition active transport, inactivation as mentioned above and other factors have an influence on the molecule flow.