The Reference Desk/Possible continuity error? Please explain!

I just watched Season 4, episode 16 (water monsters, 10th Doctor tries to change time, almost succeeds, woman kills herself). In the end, time supposedly reasserted itself; everyone still died who should have, everyone who didn't did what they were supposed to, et cetera. But two people (and a robot) who should have died survived. While they presumably weren't too important to the continuity of the universe past that point, the deaths of all on the station were fixed. The Doctor changed that, but last time a fixed point was changed, a bunch of giant monsters appeared and tried to kill everyone. This brings up two issues. 1, why didn't that happen here, and 2, wouldn't the Doctor know that giant monsters at least SHOULD appear, and avoid changing a fixed point because of that?