Official comment
You don't need to uninstall the 750. If you never plan on using it again, you can remove them.
Yes, you need the 900 drivers, the utility comes as part of the audio driver package.
Unless you're using a CDJ or XDJ, you don't need those drivers or utilities.
If you have troubles with them, be sure to have your Windows 10 fully updated and run the driver installation in administrator mode.