The over-under approach to rolls based on size (where certain things get harder/easier as your size changes) is a clever approach to handling challenges, as are the split rules for mitigating damage (which can result in the creation of new potential challenges with your dopplegangers/split slimes.)

This game would definitely benefit from some additional guidance regarding facilitating challenges:

•  I didn't see anywhere that specifies whether it uses a GM or not (I could see play happening either way, but then there are some questions about how the story/challenges get structured and how the amount of damage dealt is decided.)
• Some advice on creating an adventure and what sort of challenges a slime could face would be beneficial (I think you originally had some on the draft) but that would be hard to fit in with the limitations of the one-pager space.