I am a newbie with a 20 gallon planted tank (Java fern, Xmas Moss, Vals, Anubias, Lobelia, Amazon Swords) under low light with a 24 watt T5 HO lamp with reflector. I noted some positive results with Seachem Excel, Iron, Comprehensive and figure I could get even more growth and less algae by replacing the Excel with CO2. I considered yeast but figured to avoid the long-term hassle and go with pressurized CO2. Am I on the right track? Would CO2 this way be overkill for my low light setup? Would paintball or standard tank be better? I'm leaning toward using a standard 5lb tank to avoid frequent fill-ups. Thanks for your help.
From your lighting, I would say that you do not have a low light aquarium (but it depends on how high up you have your lighting).
You are along the right track that CO2 would be better than Excel in terms of providing a carbon source for your plants. And indeed, pressurized CO2 is less of a hassle than DIY CO2, despite the higher initial cost.
CO2 is never overkill for any aquarium; it can be beneficial even in low light aquariums, and if you decide to make a change to high light in the future, you already have the necessary equipment.
As for paintball versus standard cylinder, I always recommend to get the largest cylinder that (money or space) can afford. Larger cylinders means less time between refills, and refill costs for a 5 pound or a 20 pound cylinder do not usually differ significantly.
There are many ways to pressurized CO2; some like to buy commercially available setups, some buy from users such as myself, Bettatail or oldpunk78 that build setups, and others like to go the DIY route and piece together a setup themselves. For more information on DIY pressurized CO2 (or for general information), take a look at my pressurized CO2 article (linked in my signature).