Compatibility with early 2015 MacBook Pro and external USB-C drive?

YES.

but it will not plug in without a really simple adapter, USB-A male to USB-C female (rated for USB-3 speeds for best results)

Do NOT install the drive-maker's drivers, but instead ERASE any new Drive by its immutable manufacturer-given Hardware-device-name. This allows you to access the drive even when the drive-maker's drivers are not loaded, such as in Recovery mode.

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Yes. You will need an appropriate cable or adapter. They are not expensive.