Many pattern directions are made with the assumption that you have a 6 or 6.5x24" ruler -- it is very versatile. I began quilting when I was still quite young but right before the rotary revolution and learned a lot before the specialty rulers were invented. It was a common magazine article or the entire purpose of books to teach us different ways to get our desired results. I found I prefer the 6.5" width as it allows me to easily cut 6" finished alternate blocks. It is a really justifiable/needed expense in my book. With that and the square you have you can go far. I do like a 2.5" of some sort as well but that can wait.
I do love me my tools and I love having a wide variety of sizes of rectangles and squares, but find I very rarely use specialty rulers -- mostly because I don't remember how to use them and can't find the directions. But it is pretty amazing what I can do with a couple of angles and a grid! I do believe in things like acrylic templates if you have to cut something like fan or wedding ring or other odd blocks.