From experience, Hydra exp is rather high. I suggest strengthening the spawn by adding 10-15 upgrades to make them more worthy of the exp. Increasing HP wouldn't hurt.

With L3, as in like the Engineer, once you can use L3, you can also use ur scv to build a Riot Cannon

Similarly, once you've upgraded to L3, without actually using L3, your spores will start spawning guards.

EDIT: OF COURSE there'd be new heroes