Can I use a Mouse with my IPad Pro 2015 version 12.9 screen size

iPad has limited support for a basic “scroll-wheel + three button” mouse. If you have a more advanced mouse, that requires installation of specific driver software, the mouse will only provide basic functionality.

Use a Bluetooth mouse or trackpad with your iPad - Apple Support

If you have a Lightning to USB adapter you can even use a wired mouse.

I have the same 2015/2016 iPad Pro model as you and I've tried using an older, wired Apple mouse and it works just fine with my iPad Pro on iPadOS.