I believe there is a throw away line in Nemesis that states the Remans were used as ground troops for the Romulan forces during the Dominion War IIRC, therefore Starfleet has known of their existance for some time if we allow for that line.
Granted, I could be mistaken as I don't recall with certainty whether I heard that in the movie or read it somewhere.
Though it does make you wonder if the Remans fled Romulan space and asked for sanctuary and refuge status in Federation space, what types of tension that may cause.
Regards,
CKV.
"It is our mission to push back the darkness from the light and expand the boundaries of knowledge and understanding. That doesn't mean exploring every pleasure planet between here and Andromeda XO."