There may be "restrictions" on NPCs due to planets being shielded (Endor, Kiffex, etc) or because of rarity in existence or faction-limits (Bomarr Monks, Stormtroopers, etc). However as Elijah said, many of the races that are restricted simply out of planetary-shielding may and will still show up in the random hiring of NPCs from other planets.
I am upset things like this happen. I get upset over every godamn thing. ~ Any given SWC player on any given day.