I was wondering it that is to many for a cutter that size in an RF45 type mill.
I don't think that's a big issue. You don't need to use all the inserts. You could choose 2, 3, or 6 which would all balance. In fact, you could even use 1 like a fly cutter if it doesnt vibrate too much. Or even add a piece of steel to balance it on the opposite side.