That rinkydinkelectronics library is not a nice library. It uses its own software I2C bus for the DS3231. I doubt if that will go together with other I2C devices.
The 'SDA' and 'SCL' are defined for your Uno by Arduino. They are the labels for the hardware I2C bus.
I suggest to use a normal library.
Do you want to use a "time" library, or do you just want to read the time now and then from the DS3231 ?