First off there cant posibly be scrabs because each level wont have the big enough space as in ONI Alpha Site..if you look at the pictures of the firefight levels then you would see they are too small for a scrab... but on certain levels there can be wraiths, choppers, ghost,and banshees. So every hundred levels they will probably have a crap load of vehicles,golden brutes,suicide grunts, and Cheftains.
Also Bungie confirmed that there will be no flood in the game.