A type B will trip at 3 to 5 times its rated current where a type C will trip at 5 to 10 times rated curent or In. You can use either providing that they will comply with the required disconnection time in the regs. Though on a domestic installation type B is usual, I always use type C on lighting circuits to try an stop the tripping when a lamp blows.
Yes they can be mixed within same consumer unit - subject to the disconnection times as stated. I always now use Type C's for outdoor power sockets where they are liable to take heavy motor loads with high starting currrents that can trip Type B's.
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