In the medieval polish you can have mixed units of knights and other cavalry.
It is optional to give the Knights melee expert. The none knight elements cannot have melee expert.
Can I give the Knights melee expert in mixed tugs as long as I upgrade all the Knights in that tug?
Yes.
Yes as the general rules for armies says:
"Unless specified otherwise, if an optional characteristic is taken, it must be taken by all the elements in the UG for which that optional characteristic is available."
Emphasis added.
As the melee expert is only available for the knights it is allowed that the unit can have the non-knights without it.