Is the flange bottom fully seated on the floor? Any modern toilet should fit with any flange if it is installed tight to the finished floor. The flange you are using can distort some if it isn't seated down far enough or if the drain pipe was too long before attaching the flange. If you don't have the screws in the flange attaching it to the floor, does it raise up? It should sit flat across the flange. Are you trying to use a wax ring with a plastic horn? Did you try it without out one? Will the toilet fit without a wax ring without rocking?
The more I read here, the more I think Kohler lost everything except their price long ago. None of this probably helped, sorry.