[Pathfinder] Admixture + Prefered Spell + Rime Spell

[Agnostic Paladin]

If I'm reading these correctly (Admixture is the variant 1st level Evoker ability in the APG, Preferred Spell lets a prepared caster spontaneously cast one spell, and Rime Spell has to be applied to a [cold] spell), then a Wizard 1 / Magus 2 should be able to spontaneously cast his shocking grasp as cold damage (or fire or acid) and then add Rime to it once he gets that feat (also at level 3 if he's human, level 5 otherwise).

[Vicerious]

Yes, this works, as Preferred Spell allows the caster to apply metamagic at the time of casting and Admixture allows the caster to change the energy type at the time of casting.  The generally accepted practice when multiple effects apply at the same time is that the controlling creature gets to choose in which order they take effect.

You would not, however, be able to do it at 3rd level, regardless of race, as Rime Spell increases the level of a spell by 1, making a Rime cold shocking grasp a 2nd level spell.  Wizards cannot cast 2nd level spells until wizard level 3, and magi cannot cast 2nd level spells until magus level 4.

[Agnostic Paladin]

I'm considering the Magical Lineage (Shocking Grasp) trait to be a given for nearly any Magus.