Are od and niż interchangeable when working with comparative adjectives (i.e. ___-er "than" something/someone)? If not, what is the difference?
Yes, they are interchangeable, but remember that they take different cases: 'od' takes Genitive, 'niż' takes Nominative, of course mihxal is right I don't know how I wrote something else.
Can you also have 'I am stronger than you are'? It's a less literary, more natural form.
Well, I would go with: Jestem jak ty, tylko (że) silniejszy. Or Jestem jak ty, ale silniejszy.