Do you always keep the first Boffs they give you? There's usually never any Nausicaans in the starter pack..
You can dismiss the BOFF, but you loose it for good. I have tried to save them to use for traininng, but there is something in the scripts that prevents this.
As for the Naausicaan BOFF, you can either purchase one from the BOFF vendor, or you can wait for a reward. Most of the Nausicaan BOFFs I recieve have been Engineer followed by Science. I rarely get TACs for some strange reason. I would think they would be the most common type of Nausicaan serving in the fleet.