I've been mostly using them as a second army all by themselves, with horned serpents, bog beasts, and vine creatures as a backup. Any leader can lead them, you just have to put him in the far back and make sure he stays there, either by casting spells or by firing a missle weapon. The only problem in the early game they might have are massed crossbows, because the hydras are slow. Anything else will rout very quickly and usually without inflicting any casualties. I had a fun battle when I was defending the province with my toxic army that posioned my entire local defense militia force on the first turn (militia guys moved right into the cloud they left behind), and they routed right after that.
Btw, another great toxic combo are serpent dancers and poison slingers. Dancers die, but not easily due to their high defense, and with fanaticism you can take out some pretty serious opposition, since pretty much everybody will get poisoned one way or the other. Even though slingers tend to run into their own clouds and die as a result...
[ November 11, 2003, 17:39: Message edited by: HJ ]