We've never seen any in canonical sources. I'm not sure about the novels. As has been pointed out before, it doesn't make much sense for them to be a "star empire" if they don't actually rule over any planets with other species. But Trek likes to give us interstellar empires made up of only one variety of alien.
Based on what was clearly depicted, the only non-Romulans under the rule of the Romulan Empire were apparently the Remans, who were retconned into the picture out of thin air much later. Then they were nowhere to be seen in the very next Romulan appearance in the prime-timeline part of ST09. So apparently they decided to keep to themselves on Remus after all the hubbub with Shinzon.
Kor