Just something to consider...

If you are interested in changing the composition, would suggest not leaving the empty spacing on the right side, as the previous posting suggested. This does not have the look to me in being intended to hold text, which could be used there...

Want to keep the viewer's eyes framed into your scene, not flow from the top off the image at mid right, i would assume.

Another suggestion would be to leave the heavy fellow back about where he is, with head juxaposed against the cloud's. The remaining characters, with the tallest still in the rear but angled more towards the viewer, and closer to the right edge, body facing inward. Remaining characters more coming forward - with the dark skin one the closest and body cantered so that you can more easier get away with adding the highlights you mentioned. Even the darkest skin, when there is sweat, is going to give off highlights - and at least personally - finding satisfactory means of keeping details apparent with dark textures is a difficult trick to pull off. Have to over compensate - because jpeg conversion will undo just about anything else that's done...

Still applaud the work done on lead character and cloud. Hope this post will give something to think on,
best, hewsan