Just so you know, the current version of GRM seems compatible thus far with V1.9 of GC3. Hurray!
Horemvore has a significant thread going on using both GRM and his awesome Star Trek mod at the same time. I don't think the minor races actually interfere with each other, so you might be good to go.
If you want to remove GRM minors, go to the My Games directory, find your mods folder, go to GRM, then "game" folder. Take out the file called 0-GRM FactionDefs.xml
You can always add minor races by using a file like the above. SO if Horemvore used such a method the files will be compatible. The incompatibility would occur if both mods have a file called FactionDefs.xml
WHICH I DO, for the sake of replacing the Pirates and removing the original minor races. That is in the GRM-PRT/Game folder, and you can also try removing that file. That will result in all GC3 original races losing GRM ability traits notably... but you probably don't care about that.
PS: You can likely ensure compatibility by following my instructions for removing GRM-A and GRM-PRT in the "how to use optional files" readme in the GRM installation file.