Steel against a steel is fine, done it before.
Fine maybe, If it gets you out of trouble,But not correct
A clutch is designed to have x amount of plates, any more or less to make it work means something is wrong
The only exception to this is some aftermarket kits have one each extra but they are all thinner to still achieve the same stacked height, Only available for moderns I think though,something to do with giving better feel or hookup from memory
What may tell us more is details such as, Did the clutch slowly start slipping over time when previously all was good & untouched or has the motor been apart & rebuilt or the clutch assembly been removed & replaced or any part of the clutch from the lever down, one example is, Some bikes have a ballbearing inside the mainshaft between the pushrods if it is left out the clutch will not disengage, If the ball is too big it will hold the pressure plate too far out & the clutch will slip,I have also found two balls in there in some motors
, Only one of many things it could be though along with what others have said