Transmutation is my favourite wizard school and recently I've been thinking a lot about playing a Thassilonian schooled wizard from the Greed path. After examining all of the "form" spells such as beast form, monstrous physique, form of the dragon, giant form and various polymorphs I have observed that the details of the spell are very vague. When taking the shape of the desired creature do you keep your own stats and modify them accordingly? Or are they adopted from the creature and then modified? Do you gain the abilities of the creature (combat ability, spell-like etc.) and not just the movement capabilities? I love the idea of a wizard that can either augment himself through magic or adopt another creature's form to become a shit-kicker in combat but I lack the clarity needed to envision how spells that could bring this about are used properly. Any help here would be greatly appreciated!
You use your own stats, then modify them according to the specific spell as well as the more general rules set out in the polymorph subschool.
Perhaps one of my characters, Nudel the Devourer, will help you to understand the shapechanging rules better? She's a druid (capable of dealing 311 damage in a single melee attack automatically), but the rules for Wild Shape are almost identical to various polymorph spells. The character sheet has stats for both her humanoid form as well as her preferred form--that of the Behemoth Hippopotamus.