Well, I am playing MA Caelum in the same game Kristoffer's playing MA Mictlan (Armadillo), and I suppose my success may be partially behind why Kristoffer thinks MA Caelum needs a reduction in power.
I am also the one who got the draw in a MP who Baalz refers too playing LA Caelum. I have never played EA Caelum as yet.
My opinion for what its worth is of the 3 eras, MA Caelum is the weakest, LA Caelum has alot more going for it and EA Caelum is the strongest.
In the Armadillo game where I am playing MA Caelum I have been hugely lucky in everyway, none of my opponents I have faced have give me a major challenge. None tried to counter my powerful air magic by spending 5A gems making rings of tamed lightning or copper plate. Even MA Man who has powerful air magic himself. I have also been very lucky in magic sites I have found, giving me easy access to every shpere of magic. I would have been in serious trouble otherwise, MA Caelum only has Air/Water really. Finally I have benefited by having strong reliable allies who are happy to trade magic items with me.
So MA Caelum in Armadillo is only a yardstick to measure the nation by when pretty much everything goes its way.
To directly answer Kristoffer's question, yes Mammoths are the easiest way to expand early on, at the start of the game you have few other options.
Strategic movement/flight, those are Caelum's stongest abilities by far but only come into play mid game onwards. Caelum of all eras is the ultimate raiding nation.
Cold? Not sure what you mean by this.
Easy to get SR on a anti-Caelum SC? Very easy, 5A gems and a 1A mage will do it everytime with little to no research. Your gargoyle SC's/thugs with there 100% resistance to everything scare me silly kristoffer...
So I think restricting High Seraphs to capital only would be a major blow to MA Caelum, making a ok nation at present a whole lot weaker.
Finally, I frequent this forum alot and I have never read a thread complaining about any of the 3 era Caelum's. The only threads I have read about Caelum is how to play it correctly. New players tend to recruit the flying infantry/archers then cannot expand as it is too easily destroyed by even indies.