[Endoperez]

1) Yes, although it's actually a BIT more complicated. It always starts as that, but it can be increased further by building temples. Every 5 temples you have will increase the limit by one, so 4 dominion and 5 temples = 5 sacreds, 4 dominion and 10 temples = 6 sacreds, etc.

2) MA Vanheim sacred unit is called Van. Vanheres are the EA Vanheim sacred units, which are berserking infantry; MA's Vanir are sacred cavalry with a spear and a shield.

Other than that, yes, MA Vanheim uses Vanir and some Valkyries, MA Ermor uses Shadow Vestals, MA Mictlan uses Jaguar Warriors, etc etc. Sacred units with low resource cost and good survivability (due to glamour, etherealness, second shape or some other similar ability) are easy to rush with.

3) I don't remember the costs ATM, but what you see in the game is what they cost.