Ah ha, a MA Pythium question...

[Folket]

I find that Hydras are a powerful unit. If expanding with national units they probably are your best option since most your troops are so resource heavy. On the other hand they cost lots of gold and will delay construction of more castles. Still I would probably recruit 5-15 early on to get a good amount of provinces. In an early war hydra only armies is unstoppable by most armies.

The hatchlings on the other hand are to weak. They die on droves and still cost you 75 gold a piece.

In general hydra armies are easy to use. Put them on the front and use no other melee units.