From my experience, 10+ hours of VS survival, there is no way to select which zed you play as. You continue to respawn as zeds as long there are zeds left in the wave and players do not currently possess that body. As you progress through the game and continue to respawn as zeds, you gradually progress through the zed tiers.
If you have played through any mode in Killing Floor 2, you have noticed that not all zed types spawn each wave. The lowest tier includes crawlers, clots, bloats and gorefasts. The middle tier includes sirens, husks, and stalkers. The last tier includes scrakes and fleshpounds. And then there is the patriarch. I don't include the patriarch in any of the tiers because there is only one in a game, so only one person gets to play as him.
You are not always going to progress when you respawn though. For example, if all of the scrakes for that wave have spawned, you will not be a scrake.