I know of the creation rule that characteristics cannot exceed "5" - but what about after that point, can I go to str/agil/tough/int/fel/wil etc of 10? 15? 20?!?!

Im sure I missed this somewhere

No, any future advancements need to be listed on your open advance section and since there are only 6 of these, six is the maximum characteristic you can boost any of these stats to.

Yup, it doesn't need to be said anywhere since the advancement mechanic limits it to 6.

Except for an Ogre, they get a benefit to allow to buy certain stats cheaper so can be 7 in that stat.

Haven't the Ogre book here so cant give exact details.