[Ironhawk]

Generally speaking, no, poison summons are not worth it. There are exceptions to the rule, of course, but for the most part, no.

As for national units: I have used the pythium hydra to great effect as a solo small squad (3-4 hydras) early expander. They are quite good for that. But as the game progresses you have the same problem of not being able to easily field masses of poison res troops to support them.