Well, seeing your project is looking very good so far I would be willing to create a custom rock model for you. The Rock(s) would come with 3-5 variations having 1 custom texture.
I think the custom texture can range from a square 128 to a square 512, depending on how much room you have for an import and how much qualitiy you want to have within the texture. However I guess 128 or 256 would be fitting perfectly.
Each model would take a filesize of ~6-10 kb, the texture would take up between 40-140 kb depending on converting prodecure settings.
Do you want me to create such a set of rocks for you?