in my experience, you can have a patch antenna excite a standard waveguide horn. It works ok. It is especially useful if you need isolation between multiple antennas, as in a cw radar with a transmit and two receive horns. If you just had 3 patches on the same board, the leakage between the patches would be too high.
But the reality is that to get good high gain, you need a very long horn with wide aperture...not that conducive to mounting on a pcb. I have had good luck laying a dielectric lens on top of a patch antenna to get better gain. You might want to go in that direction.