Yes, you can apply different tariffs to individual clients. There are no limits to the number of tariff tables allowed. You can create a completely bespoke tariff for any client or you can just edit an existing tariff and apply a new name.
Yes, as all tariffs are completely customisable, you could zero-rate calls to a specific number or destination. By saving this edited tariff under a new name, you could then apply to certain clients only.