I don't know much about colors or textures yet, but if you're shooting for a "fuller" feel with the body, my main suggestion would be to try and block it out with geometric shapes (like spheres and cylinders) first, to get a feel for where the mass should be.
Sorry I can't be more help :/
I like it! The one nitpick is that usually in the show, the irises are set so that they touch a corner of the eye, rather than just floating in space. Otherwise, I think it's a very cool crossover!
My main thoughts are that RD's front (stage)-right leg curves out a little too much at the bottom. Also, I can't figure out how Scoots is supposed to be laying; but maybe my brain just isn't deciphering the body lines properly. Other than that, I love this pic and the idea behind it, and if I had any money I would give it to you to make more!
And now for my own doodles! I've been drawing some Canterlot Royal guards to try and figure out something about pony armor, so here goes:
Spoiler