So, I just tried to create a new character on other builds, and for some reason the Twi'lek is available for Sith Maurader, Inquisitor, Bounty Hunter, Jedi Knight, Consular and Smuggler, but not available for Imperial agent or Republic Trooper.
And yes, I did use a boost for my level 75 Twi'lek. But it does not matter, because I cannot create a Twi'lek Imperial agent or Republic Trooper, which I have on the other servers.