Hey tmf33 good question!
You'll want to be cautious of this because if you downgrade your plan after and there are any changes made to the product or they need to be updated they will fall out of sync. If you go through the forum you'll see regular occurrences of needing to re-sync products. Again, up to you to take the risk!
However I would highly suggest starting with the limit of 500. If you can't sell one product there's a good chance you can't sell the other 499 or 9,999! Once you get started and have sales rolling you can easily upgrade and include more products. Most successful stores only have 1-3 products that bring in the most revenue.
Up to you of course as it is your business but something I would consider as you go forward!