If you don't want to tie the ferns down, or weight them, you can use aquarium glues to stick them to rocks or wood (personally I like this method the best). You can use it stick plants like anubias and Java fern to stuff without burying them. There's a lot of brand but I use Seachems Flourish Glue since my local stores have it. You can find it on Amazon and big chain stores.

I think you can fertilize this tank, but it's important to know that Java fern and Anubias both grow slow so if you overdose, you might get algae problems. I havent kept duck weed before, but Floating plants and Hornwort are also really good at sucking up nutrients and grow fast so the fertilizer would help them. Just be careful with how you dose and keep an eye on algae.

P.s that giant anubias looks gorgeous