I believe there is an interaction between the subject and the format ratio when deciding on the composition. Sometimes the subject dominates, and regardless of the format ratio, you know a particular subject demands to be cropped into a square picture or a long thin one.
But most of the time my instinct is to "fill the frame" and I find that the geometry or spatial weightings within the frame are different with different format ratios.
For example, with square pictures, I often end up with the horizon running through the centre of the frame (not always) but hardly ever do this with a horizontal or vertical rectangular format.
Taking horizontal pictures with 5x4 means you need to think much more about what is going on with the top and bottom of the picture than you do with a 36x24 format, which is much longer and thinner than 5x4.
This means that if you only consider the horizontal angle of view, and use equivalent focal lengths with 35mm and 5x4 formats (or square) a composition that works with 35mm, may not work with 5x4 (or square) because you are including so much more of the vertical view with the 5x4 and square pictures.
It also means that in practice, "equivalent" focal lengths between formats aren't actually equivalent focal lengths when it comes to composition.