Sure, no problem ! - I think once you start upgrading the quick zombies they become pretty over-powered. I think maintaining a more challenging balance with their health would've made them work in combination with the other zombies better. I guess it's quite difficult because a big element is the speed of each zombie and trying to time their positions on the map with each other - maybe if there was also a way to be in more control of the zombies individual speeds - that might also work as a power-up that could add a layer of player control and tactics to the gameplay