You can try changing the gamestage number for each type of zombie to see if they spawn in later. All of them currently are set to 1 so I think it is just probability and internal code that determines when the harder zombies spawn in. If you change it to say 50, that might limit it to spawning in at GS 50 or later (not sure, like I said never tried it before).
An easy way to try is maybe changing a lot of the basic zombies to a higher level and seeing if they don't spawn in early game.