Most of the time, they request you print both. And, yes, you can just switch out legal for the letter. But, documents must stay in the order they were sent to you. So it can be stressful, especially when you are just starting out, to have to worry about switching out pages.
Look, this is a business and a very cheap one to get started in. If you want to be professional, do the best job you can do, and in this way drum up enough business to live off of, then you need to put forth the money to purchase professional equipment. I just purchased a printer for $300 at Best Buy (online). It is the one I use in the car because it is fast, compact and has beautiful print. The one I use at home, which was the first one I bought (you don't need to worry about being mobile as yet.....start making money with your home equipment and then graduate to a mobile setup.
Now to make it a cheaper setup, I would get that same printer I mentioned previously and use it at home to start. It is the cheapest way to get started with two trays. And, by the way, I did reprogram the trays. Now it prints on both legal and letter automatically. Although HPs aren't my favorite (they are cheaply made and break down within a year or two, I've found), but they resolve your start up business for sure!