If I subscribe to "Epix", will I get that subscription on both my front room "Ultra" and my bedroom "3"?
How about other "premium channels like "Starz"?

Yes. If you subscribe thru Roku's system, your subscription will work on all Roku's signed in to that email account.

If both Roku players are registered to the same Roku account, the channel should appear on both of them. I don't have Epix, but I would assume there is somewhere within the channel for entering your credentials. Unless that subscription is per device, you should be able to log into their channel/app on any device, Roku or otherwise. 

When you add any of the Roku channels to your Roku account, they should show up on all Roku units that are registered to that account.  (Exception: some channels may require the hardware capabilities of later model Rokus.  For any such channel, it will not show in older models that don't have the required hardware capabilities.)

You generally will still need to log in one time with EACH Roku unit to authorize it for each new channel.

As far as I know, you'll be able to watch on all of your Roku enabled devices. For Starz, the only limitation that I had was that I couldn't watch on more than "4" screens. I suspect that this is because I had a direct subscription through their website and not through a cable provider. I'm thinking that you should because with cable you can watch any channel on any number of screens so long as there is a cable cord hooked up to the device, yes? 

Also, for Starz, I had to go through the whole process to activate Starz because it kept asking for a code. Maybe that's how they limit the number of screens. Let me know about your experience. I could sure use a workaround.