Well... Clownfish don't actually need that much space. They spend their whole life in a really small area in the wild, they don't swim around or wonder far from their host anemone.
I guess the problem would be keeping the water quality satisfactory.
If it was my tank, I'd stick with some small goby or two, couple of sexy shrimps and a Lysmata amboinensis, couple of hermits and snails.
Gobies are great nano fishes because you don't have to feed them as much as the clowns. They eat whatever they find from the substrate and rocks, like small critters and pods etc.