Unfortunately, the fact that they can spawn does not mean they do spawn.
It's kinda hard to apply any sort of tactics when your way is constantly blocked by low level monsters. With no way to get them out, your only option is to kill them, and this usually results in you having to kill too much popcorn too early. Not only this is bad in general, it leads to Rex starting to hunt for you when you're level 4. Which also means that you are nearly guaranteed to become trapped between him and something sooner or later. Considering that while the boss is not the exact definition of "weak", this is usually fatal.
You can probably tell that I'm not fond of maze levels.