Official comment
You could use the XDJs with a USB hub, as they're only controllers. You don't need the ethernet adaptor as you're not using them linked (export mode), and you don't need the aggregator for the XDJs as you're using the DJM-850 which has a multi-channel audio interface.
Connect the XP1 and XDJs to the hub, connect the DJM directly. Configure rekordbox to use the DJM-850 in external mixer mode, sending each deck's audio to a different channel on the mixer.
Boom!