Mrs. Johnson definitely is NOT a Leviathan. There's a line that I am afraid I find wickedly hysterical when the Leviathan Leader (don't you wish they'd given him a title, something along the lines of, oh, I don't know, The Great Naga???) babbles something like "And I'm gonna kill that housekeeper... that Mrs. Johnson. I never could stand her!"
Paul is not a Leviathan, either, and proves to be rather a thorn in their side. This plot thread was dropped when Dennis Patrick had to go make a movie.
Barnabas explains at some point that certain people have a "genetic structure" which resists absorption into the Leviathan consciousness.
Gothick