I've had fun with almost a thousand fleas in a private lobby once. slowed game speed quite a lot, moving them all randomly, or even not moving them at all. once you get a critical mass of fleas, they can chainsplode to some extent. the new engine actually fixes the bug where they would crash into each other when mass moving for near instant death, but it is funny to watch about 300 fleas grinding their dead explosive bits together at a light lazer beam.
It is also fun to rush fleas in 40-50 mass in the beginning of a game to shutout all eco and melt a factory or com.