Can I mix TMC drivers with A4988 on the same RAMPS board? The closest thing I can find is this wiki page that state DRV8825 and A4988 can be mixed. However, the TMCs seem different, requiring more connections and offer more features, so I wonder if they can be used on the same RAMPS board with A4988, particularly for X and Y axis.
Yes you can mix different drivers, including the TMC drivers (e.g. using for X and Y only).
How you do that is described in this instructable.

Please do note that, from observations, the 8-bit based Arduino boards and shields such as the RAMPS are becoming more or less obsolete; the 32-bit based controller boards are becoming mainstream. Such boards have a lot more potential in execution speed, memory and more available options for peripherals.