Actually, I think some comics vary the shape of the text boxes. Sometimes a circle, sometimes a rectangle, and sometimes a spatter pattern. The elipses are fine in moderation. Squares for everything could get just as boring which is why I suggest that you experiment with different shapes and possibly box colors to convey different moods. Too much of one type may dominate the frame. An occasional thought bubble is fine. Narrative is sometimes distinguished from thoughts by the color of the box, parentheses, or italics. Next time you're at the store, try thumbing through a comic book and pay attention to things like the text boxes and some changes in perspective. Also pay attention to the color scheme and see if it matches well with the story. Good Luck, you're off to a really great start.