I RECENTLY PURCHASED THE QU-16, BEFORE I WAS USING A BEHRINGER XR18 with the personal P16 mixers for IEM mixers… can the P16 be used with the Qu16 mixer?
No. The Qu uses a proprietary IEM system. The Qu does have apps that allow musicians to control the mix on the monitor outputs.
A&H has a similar system to the P16
it’s called ME system 
https://www.allen-heath.com/key-series/me/
in the end these devices are personal mixers, they get fed with splitted input signals from the desk
but the protocols used to handle the input channels are different
the ME system supports up to 40 inputs