Idk. Rego is useful to make the ice move and to help distribute the weight without a need for padding or leather straps, but I don't see it doing a whole lot for complexity, and I'm not sure the extra magnitude to go from base 3 to base 4 (natural to unnatural shape) is enough. But well, that's a minor point.
For protection against cold, you are right, Rego is not enough. It's probably easier to just invent a ward against cold and have it ready if the need to cast this armor on a mundane arises.
Rego reduces the need for complexity. The Silvery Scales need to be designed to minute detail to avoid encumbrance, and it moves only because of the wearer's muscle power. The Ice Armour moves magically (by Rego) to reduce the load on the wearer, and even when it does it hampers the wearer a lot more than regular armour. That's the difference.
With two magnitudes of complexity, you could make Icy Scales with the same expert design as the Silvery Scales. Fitting perfectly, with the encumbrance of scale mail, but unfortunately the scales would break on first impact and not offer the same protection as Terram.
It is right that the cold/warm part is Ignem, but cold is the natural state of ice, and Creo is supposed to create things in their natural state. I tried searching quickly in the book without finding anything solid on the matter, though, so I suppose one could conceivably build a case either way.
Another matter which I considered was if the cold, as transferred from ice to body, would be natural and non-magical, and thus unresistable by parma. Without any confidence, I concluded that that would just be making things difficult with no narrative benefit, so I left it.