Hi there! I’m also new to the forum, so hello and welcome! that’s a pretty nice setup you have, and congrats on the reef—something I’ve wanted to try for years but never had the guts to spend the money on.
As for stock ideas, it seems like you’re working with lower light. Bettas enjoy lower light, not to mention how gorgeous they are, and they typically leave bigger snails alone if you wanted to add some cool ones. Since it’s a 16, you could try a ram or an apistogramma, but stick to a solo fish or maybe a pair. If you want activity and volume, a school (10-12) of small tetras or rasboras would look nice as well with a few corys or otos.
I have a 14 cube with a pair of apistos and 7 X-ray tetras and everyone gets along just fine since it’s planted.