Thoughts:
1. 500 words means just that, no more. It does seem very brief, sure it wasn't 1500? That was our standard essay size. Whatever, stick to the word count, not the pages. One of my lecturers would state 'no more than x words' stop and refuse to read even to the end of a sentence!
2. If it is your own thoughts then you don't need the ref. but if it's eg 'Piaget was known to refer to this system as blah, blah' (Then you need a quote for the info) Harvard is the sytem of refs - ie how they are set out, not which bits need references.
3. Just answer the question. You can do an intro based on what you are about to say and a summary, but in 500 words that's going to be VERY brief so you can probably skip it, especially on a comparison cos you don't need a conclusion by the sound of it.
4. You're going to be tired tomorrow! Good luck