Off the cuff I'd work it exactly like the various summons spells - several series of spells with different XP caps, each series lets you turn into only one general type (there wold be a polymorph to animal series, seperate form a polymorph to construct series). I'd make the player keep their own mental attributes and vitality point total in any form, and I'd give the critter stats they are borrowing a -2 Threat level, but allow the character to take weapon proficiencies (1 per form) to really practice up with that form, bringing it up to normal threat level.
And if they are using the spell to disguise themselves as someone in particular, make a Disguise check. The spell counts as a kit and might allow them to ignore certain penalties like shifting hieght and weight, but their attention to the details and nuances required to impersonate someone is already clearly described on their sheet by their disguise bonus

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