I haven't seen anything on how they were assigned, but given that for most of the war period housing was in very short supply, and worker turnover was quite high (at Hanford average monthly turnover was 20%!), there was probably not any great system to it so much as making sure that senior people had better digs than those lower on the totem pole (as was the case at Los Alamos and Oak Ridge).

The best source I know of for the minutiae of housing administration at Hanford is the Manhattan Project History that was compiled shortly after the war. The DOE has recently declassified many volumes of it that were previously still secret and put them online. The two that seem of value to this question are in Book IV (Pile Project), Volume 5 (Construction) and Volume 6 (Operation) In both you can find sections on Richland Village, as they called it. I skimmed the Operation volume but didn't find anything specifically on assignment of housing, though both the Construction and Operation volumes do emphasize the haphazard nature of the Village. They also both emphasize that this was handled by subcontractors who (I am guessing) had no knowledge about the content of the work being done by these people — they would only know their pay grade and probably that was all that mattered.

As for security personnel, I don't know of cases of Hanford employees being assigned personal security personnel but I wouldn't be surprised to hear that a few were. To my knowledge, at Los Alamos only a few of the very top scientists (e.g. Oppenheimer and Fermi) were assigned bodyguards/chauffeurs who were required to be with them at all times. (There is some discussion of Fermi's bodyguard in Laura Fermi's Atoms in the Family — he became a close friend of the Fermis.)

Hanford used to have a pretty great document archive online that might have materials relating to this but it was taken down late last year and has not yet been restored, though I have been told they are planning to eventually restore it.