BE2012 CASO

I've BE2012 at Main site and BE2012 at DR site. Both servers are standalone Media server.

Both servers didn't join domain. Both servers've connectted OST Storage device(NAS device) in each of site(network can be recognized).

I would llike to create CASO but I've question:

1. CASO requirement needs join domain both media server(Main&DR site).  What's effect to media server after join domain? such as need something change the configuration or effect to OST device?

2. How to enable CAS and MMS which already installed stondalone media server?

3. Can CAS and MMS be different OS version?