Nah, Tetra's don't mind the extra flow and some species actually enjoy it.
You don't need to get a filter rated just for 20 gallons if you have a 20 gallon tank. And actually it's recommended that you get something that's rated for more gallons, usually twice the size but a filter rated for 30 would do you great! I prefer the AquaClear 30 if you can get them, Aqua-Tech kind of sucks, I have one. But the AquaClear has different settings for flow adjustment as well which is nice and it it doesn't have cartridges, you can customize your own media. Usually you use the AquaClear foam, carbon and bio-max packages but I usually skip out on bio-max and use two foam's instead with the carbon between them
More foam means more space for Beneficial Bacteria to come chill out in your tank which is good!