This is how I understand it Trill, I might be wrong somewhere along the line, but here goes. 

The Romulan's have a Vulcan origin but they rejected logic and the Vulcan way of life.  The Romulan's settled on Romulus and became a mighty empire (Romulan Star Empire).  In the Vulcan arc through series four, the Romulan's where intending to take advantage of the lack of Surak's teachings and assimilate Vulcan's into the Romulan way of life.  The Reman's are a race that live on the dark side of the moon orbiting Romulus and the Romulans enslaved them to do their somewhat dangerous dilithium mining.

I feel like a history teacher. LOL

"It's Judge Q to you....."

but I heard somewhere from a trekkie (or trekker?) that the remans and the romulans supposedly have the same origin and that they were separated because of a divergence of opinion on certain matters exactly like the vulcans. so I guess this would not be true then and they are not related. but how did they get on the moon anyway? they must come from somewhere...

No, Romulans and Remons are two seperate entities and the only thing that relates them is that they live in the same system.  The thing to understand is that the Romulan's originally came from Vulcan and arn't indigenous to Romulus.  The Remons are indigenous to Remus (the moon orbiting Romulus) and its their homeworld. 

Going back a step, you asked if Remon's had emerged in Trek before Nemesis - I think (to my knowledge) the answer to that is no, we haven't seen them until ST Nemesis, unless they have been in TOS and I've forgotten.

"It's Judge Q to you....."

You're welcome Trill. That's why I thought it would be a good idea to have a thread like this because we can settle our queries!

"It's Judge Q to you....."