I think to do this......get all of your t-shirts mounted on the interfacing.
Lay them out where you think you want them. Then, fill in the blanks with whatever else you want to make per row. Once you have your extras finished lay them down too.
Then, go back and make your row the same height across. If you don't want the rows the same height then make the columns the same width.
I made a quilt with words for my daughter. That's how I did it. I used both filler fabric and stars. My words were not as varied as I thought they would be. Which was ok. I was ready to be done with the quilt when I noticed this