Can I use baking soda or baking powder to clean my Keurig coffee system?

No, baking soda (sodium bicarbonate) is basic. Baking powder is essentially neutral as it is a blend of baking soda and cream of tartar (an acid) as well as starch, which would only help gunk up the works. The scaling in your Keurig is from minerals in your water which, when deposited and dried, are basic in nature and need an acid to dissolve the scaling. Vinegar is a dilute acetic acid solution and citric acid is, well, citric acid and is contained in lemons. Both are mild organic acids that work quite well to remove mineral deposits.