I should have mentioned this above, apologies...but since I used mine as a complication between two teams, I made my zombies in addition to above "immune to psych" (to use a term from a different game) so no morale or stress. Interesting with the TWD rule, In my game Zombies never attempted to seize the initiative, they only reacted to either teams to failed activations, and whenever initiative changed between teams I gave ALL the zombies a chance to activate. Finally because RS has zero rules on movement other than how far you can move (run, walk, sprint,etc) I use the standard 1" control zone from other games in RS, (meaning no model can pass within 1" of any other enemy figure without automatically being engaged in melee)
On a Space Hulk style board with a dozen zombies this led to some craziness, Of course OOA figures brought down by zombies will become Zombies themselves!- I'll save that for my AAR which I should get posted sometime this week.