Can I reuse my old GPS module through some other port on my PX4, or am I going to have to try to purchase one? I have three additional mini’s for my FPV drones, but they are not 6 pin either.
There is nothing generally speaking against reusing those ones.
The main question is if they are supported by px4, that depends on the model of GPS.
Then you can just rewire it to your flight controllers gps port.
GPS need at least 4 pins, GND, PWR & 2 UART. With the 6 pin you also get a I2C interface for the magnetometer, lights etc. that are also commonly found on gps modules.
And with the large 10-11 pin ones you also have things like a buzzer, safety button or lights.
Could you tell me your gps module, and flightcontroller?