I've seen sumps on freshwater tanks, but I've never understood why.
Sumps are useful in saltwater tanks because 1) Saltwater is very dense, and more difficult to oxygenate than freshwater, and 2) High oxygenation is very important to keep a high pH, which is necessary in a marine environment. Neither is true for freshwater. I maintained approximately 85 total freshwater aquariums in the field and at my business and never measured pH. I had to do bi-monthly water changes anyway - that's the only good way to "adjust" pH in freshwater. In nature, pH in freshwater systems fluctuates wildly.
You can provide adequate aeration in freshwater through traditional means, i.e. HOB filters, etc.
However - if you want to keep tubing and other unsightly stuff out of the tank, you can't beat a sump. Puffers are famous for biting through electric cords, airline tubing, fingers, etc. And the extra oxygen is always beneficial. Come to think of it, a sump sounds like a good idea for puffers!
You can use a 20 gallon, but I'd keep it very simple - socks/sponges, media (bioballs?) and a good return pump. That's all you need.
Good luck with the Congo puffer. That's one of coolest critters you can keep!