Iron Stars is a completely different game from Starmada, set firmly in its own universe, with game mechanics specifically designed around that universe.

Starmada: Iron Stars is an attempt to demonstrate how the Iron Stars ships would look in Starmada terms, adding various options to the game along the way that can be used in any setting.

Yes, you would need to "upgrade" from Starmada X to the Admiralty Edition. (The general consensus seems to be that it's worth doing anyway...)

Hope that helps!