In all reality, if you have ~7 Hydras on a flank with charge orders on them and you keep your main force farther back than you normally do and on the opposite flank, you can do fairly decently on the friendly fire deaths. But in general the armies are incompatible unless you count on taking high losses.

One way to make sure your army casts Poison Ward/Serpent's Blessing is by only sending in your Hydra combined armies against large armies, then the trigger fires for gem usage.