yes you can do whatever you want with the mesh for your own use, it is only redistribution that is not allowed. as far as increasing mesh density, while you can do that be aware that that will make all stock morphs unusable including facial morphs. Transfering UV mapping may not be possible due to the fact that you mirrored the toe group, that likely created a seam between toe and foot which would duplicate the points along that edge and the uv transfer file wouldnt know how to deal with those extra points.