I have been altering some spawn points but most areas remain basically the same. There may be a lot of ankhegs in the area just north of FAI but they live in temperate forests and plains everywhere and therefore I added a few of them to some other areas as actors, not through spawn point. As you may recall, there is also an ankheg at the top of the map where the bridge to Baldur's Gate is located. BioWare also added three ankhegs to the dungeon of the Island in the Trackless Sea that Shandalar teleports you to, but I removed those, of course. Basilisks live basically everywhere which is why there are a few of these elsewhere (also as actors, not spawn points). Besides, every BG player knows that that area is filled with basilisks and is never going to travel there without some spell or potion to protect himself from their gaze. By having a few basilisks in other areas, the player is going to experience situations that he cannot metagame his way out of.
Bassilus may be responsible for some of the skeletons and zombies in his area and the nearby ones but hardly all of them, and not the ghouls and ghasts. Besides, killing Bassilus does not stop these any of these creatures from appearing.
-Echon