Certainly you can model and copyright your own creations, even if they're modified versions of something like Stuart Little. The key, tho, is to make them adaptations, not direct copies.

Now, there are some who feel that making a mesh of something like a cartoon character (like Mickey Mouse, for example) means it's completely free of copyright because it's a mesh, not the drawn character itself. That, unfortunately, opens the can of worms called trademark, which is very different from copyright. MM, for example, is a registered TM of the Disney Corporation, and his likeness is seriously protected as part of their IP. Even as freebies, youre legally in problematic waters.

So bottom line: tred this way with a great deal of care and caution. Make sure that your "inspired by" characters have enough visible differences to set them apart from their origins, and you should be fine.

docandraider.com -- the collected cartoons of Doc and Raider