Patrol nodes should be enough to give aliens a path to follow. You can get pretty complex with them too by having randomised paths and commands and such.
You can also make aliens "lurk", which has them follow the player but retreat if they're seen. They'll only attack when they get within a certain distance. It's very creepy when facehuggers have that command.
Also, I remember once giving all the aliens the headbite weapon on one of my maps just to see how it would go... It was the most brutally difficult thing I've ever played.