Duetto filter and C02

Hm. Not sure about plants that'll stay small, but bunched plants are an easy way to keep them in check. Anacharis is a good one to start with, you can also add a Anubias, I think there's a species that stays really small. You could also try to add some java moss.

That's really all of the options you have for such a small tank.

The only plants that color nicely are the high light plants, that get big. Or some bunched ones (Rotila) But they need high light.

As for the co2, you can just stick it in the tank, you don't need to put it in that spot on the filter as it could make the filter malfunction over time

I have DIY CO2 running into my Duetto filter in an 8 gallon tank. As soon as my new glass diffuser arrives in the mail I'm going to switch. The CO2 makes the filter really noisy and, I know this sounds crazy, but it puts too much CO2 into the water. I can't keep the CO2 level down below 30ppm. This would probably be a problem for you too because your tank is even smaller than mine.