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Random level, yes.  For XP, wiki states that only blue Aetheria start out with XP, as they wanted players to be able to get to level one, which is the passive spell activates and the Surge has a chance to proc, faster.
When the coalesced Aetheria is activated, via the Aetheria Mana Stone, the string property 16 is changed to 'This aetheria''s sigil now shows on the surface.'
The Surge spell is added at weenie_properties_spell_book and weenie_properties_d_i_d 55
The Aetheria set is added at property 265.
The Icon is changed from the coalesced form to the one that has the sigil based upon the set.  The different icons start at http://ac.yotesfan.com/06-Textures/?id=06006bf2
The IconOverlay and ItemMaxLevel are determined at item generation time.  The icon overlay values are 100690996 through 100691000, which adds the 1 through 5 symbol in the upper right corner.
I preset the base weenies to level 1, but they won't necessarily get dropped as loot in that state.
http://acpedia.org/wiki/Aetheria
Aetheria
The current level of an Aetheria is a calculated one based upon the what the current ItemTotalXp is, the ItemBaseXp which is the amount needed for each level, up to a maximum level given by ItemMaxLevel.
The total XP needed for an Aetheria to reach it's max level is ItemBaseXp  x  ItemMaxLevel.
Wherever ItemTotalXp is at any one point, within ItemBaseXp  x  ItemMaxLevel and in increments of ItemBaseXp, determines its actual level.
I would imagine that every time the Aetheria gains XP, the server needs to send a property update for ItemTotalXp to the client.
I worked this out by plugging in different values of ItemTotalXp, and the assessed level of the Aetheria changed accordingly.