The KEF LS50 wireless can be an endpoint or “zone”, however when used as an endpoint they can not be “grouped” with other zones as they are not fully compliant (search through the KEF area for more info).
If you use another endpoint or Roon Ready device (I use a RassberyPi with HiFiberry Digi+ pro.) to hook up the KEF then that device can be grouped with other zones.
I will let someone more familiar with the Mojo/Poly talk about that question.
When you say “in-kind” do you mean the same equipment/manufacturer?
Because if you do, that it not the case, Roon syncs the outgoing streams and has controls for those instances where you need to adjust it.
I have multiple zones of varying equipment and have never had to mess with syncing stream to any of them, Yet…
Also, note that due to the technical details of KEF streaming these speakers are not Roon Ready, so linking multiple pairs of LS50w is not possible and some RAAT-specific functionality (like convenience switching or advanced Signal Path functionality) is not possible.
I suspect that being unable to link multiple pairs would also preclude linking to other RAAT Zones but I haven’t any personal experience. Let’s see if @support can say for sure.
One option is to use a RAAT endpoint to feed the KEFs (or the Mojo) instead of direct Ethernet connection. That would be capable of being synced with the Bluesounds.
The choice of endpoint depends on what connection and resolution you need. At the lower end there are Raspberry Pi based devices that perform very well. I use an Allo DigiOne in one system and find the sound good up to 192 kHz, but it falls over every now and then if left on. I think it is a known grounding/static issue that can be resolved by feeding it a separate power source.
In my main system I use a Sonore microRendu which does Ethernet to USB very nicely up to 768 kHz or DSD 512.