I disagree about it being so important to keep the animal and base creature the same size. All magic items automatically resize to fit their wearer by default, except for armor and weapons. The weapons won't matter much for natural weapon based builds, and there are ways around the armor problem also, like using a Monk's belt (or taking monk levels) or a Skin of Ectoplasmic Armor. If you want to use weapons, you can get the Sizing enhancement, or just carry around two weapons of different sizes, also (or just eat the -2 penalty for using an inappropriately sized one).