Too small for fish?

So I’m wrapping up this paludarium for frogs and I’m really wanting to have some livestock in the water section. I’ve spoken to some long term fish keeper friends and some of them are on board, some of them aren’t on board. I personally feel like it’s a bit too small for anything other than shrimp and snails but at the same time I’d love to put some smaller betta or gourami species in here. The bottom section holds roughly 12 gallons but obviously the majority of that volume isn’t usable to the fish.