I've zero knowledge of snooks. If they're quite a fast growing predator you are narrowing down your options because whatever dithers you get are likely to get eaten, unless you start off with already mature dithers, or you grow out some in a seperate tank until they're too big to be threatened, and then add them to your snook tank.
I've experience with balas and tinfoils, both are best kept in multiples. Due to their eventual size and skittish nature a 180 is pushing it imo for both these species. Some species of silver dollars are smaller, they may be ok for you, but i've always kept away from them because i've heard they can be super skittish.
Filaments are quite big when mature, and deepish. I don't know how big a snooks mouth is, or how fast they are. Filaments are fast but in your situation you'd have to grow a group of them out for a while, and then cross your fingers when you add them to your main tank.
You could have a really nice peaceful tank if it weren't for them pesky snooks, lol.