No matter i worked it out. Don't think it is possible without altering the main files, but i copied and created a new set (Terran2) and just use that instead from the mod folder.

Yup. Just copied over the 2 main files and deleted all from them except the Terran ships/starbases. Then simply gave them a new distinct name (added a 2 after the name, simple add) and altered the shipyard & starbase models to the ones i downloaded. All i have to do then is to go into the custom race and select the new shipset, which is default terran with new starbases and shipyard models.