Using both MOCA and Ethernet isn't a problem at all -- but you need something that speaks both MOCA and Ethernet (i.e., act as a MOCA - Ethernet bridge). The Roamio Plus/Pro can do that IF it is connected to both (which it sounds like it is). I don't think the base Roamio works as a bridge, but I am not 100% sure.
I don't have a Roamio, so I can't test it myself. But you have everything you already need:
Assuming none of the other rooms you already have Minis in have Ethernet, just "borrow" one of those Minis and connect the switch via Ethernet and see if it connects. You may have to tweak some settings:
- set the Mini's network settings to use Ethernet (not MOCA)
- make sure MOCA and Ethernet are both enabled in the Roamio. If there is any setting to make the Roamio a MOCA Bridge, enable that.
Here's a TiVo document you might find helpful (or confusing?) -- basically if you have a Roamio Plus/Pro (or Bolt), you don't need the MOCA bridge as those devices already have that functionality. Tivo Customer Support Community
One of my Minis is in a similar situation, no COAX in that room (but it has Ethernet) - since I have FiOS, my router acts as a MOCA bridge (so I am not using the Bolts for that, they are just MOCA clients), and the Mini works fine via Ethernet.