As long as both devices are running the same protocol (RAAT in this case), you can group the devices. I confirmed I can group my MacBook Pro Remote (running RAAT over its System Output or native speakers) and my Windows Core System Output (running RAAT over its System Output).
I don’t believe the WiiM Pro is Roon Ready yet (I believe this is expected this summer), so it does not support RAAT yet (only AirPlay and Chromecast for now). When its firmware is updated to be Roon Ready, it can be grouped with any other RAAT-supported device(s).
The WiiM Pro did show up in the list of Roon Ready devices but I’m not sure if that is just in beta mode. I’ll dig into the protocol on both devices and post the results.
Yes, if the streamer is Roon Ready. A few, of many, examples are the Bluesound Node, Cambridge Audio CXN V2, and Lumin’s network streamers.
Alternately, you could make your own streamer using a Raspberry Pi 4 running an audio-specific OS such as RoPieee and install Roon Bridge on it, enabling RAAT via Ethernet or WiFi.