Are we limited to just WPILib? Can we include only approved libraries? Where could I find this information?
There’s no limit…
As long as the licenses in the library support fair trade use, than libraries are fine.
Obvious exceptions include libraries that interfere with the hardware operation of the roboRIO or the driver station/FMS packets.
Any non-free libraries would presumably need to be accounted for in your Bill of Materials, and any libraries that you wrote yourself but did not release publicly prior to build season are considered past seasons’ “robot parts” and cannot be used.
In addition, it would be wise to obey the license requirements of any libraries you use (this is less of an FRC rules issue and more of a legal one!).