You have a higher chance to hire Geonosian NPCs on Geonosis (90% I think it is, rather than being split evenly with all the other races on non-homeworlds), but you can hire them anywhere. You cannot hire them specifically, which races you get is randomly determined.
As for Geonosis, I believe it is shielded, but contacting the faction that controls it (New Republic of Falleen Federation I think) would be the way to go past.
Tiali, not all homeworlds were set to NPC controlled, just those that were given up, some 20 races. The rest are still controlled by players.