It will probably work OK, but less then ideal. One of the biggest drawbacks to the H16 system is that the bottom block doesn't pivot, so you're quite limited as to what angle you can adjust the downhaul - you basically have to be right behind the mast. With the H18 system, you have more flexibility as to where you can be on the boat and still adjust the downhaul since the cleat pivots.
The other downside is the connection of the upper block to the tack fitting. The H16 system is designed to shackle to the H16 gooseneck. This won't work on the H18, so if you use the H16 system, you will have to tie the blocks to the tack fitting. The H18 downhaul system has a quick pin fitting that allows you to easily connect or remove the blocks from the sail just by pulling a pin, so it's more convenient.
I would probably return it and have them send you what you ordered.