Hello,
The omni 493 does seem very well suited for your use case and has the potential to provide performance improvement.
Using antennas with differing gain and directivity properties is not against MIMO. It might provide a more versatile configuration for receiving signals from close proximity and distant base towers.
However, radio transmission has many variables to account for, and the best suggestion would be, if possible, to first test every setup before making any commitments.
Best regards,