Does anybody know a definitive guide on combining models for batching?

I don't know of a definitive documentation for dynamic batching, but if you Google this list with "Dynamic Batching," you will get 1000's of hits. An hour of reading will give you a pretty good idea of what works and what doesn't. Here are the high points I've gleamed:

Won't work:

• If the materials are different, the objects do not batch.

• If you change any material attribute at runtime (color, texture, offset...), then the object gets a new instance of the material, and that object will no longer batch with the objects using the original material.

• If you have more than 900 vertex attributes in your mesh, then the mesh won't batch. For example if you are using both normals and uv in a mesh, then you can only have 450 vertices and have the object batch.

• Submeshes that use different materials don't batch together. Different material means different GPU call.

• If you use Graphics.DrawMesh() to draw your mesh directly, the meshes don't batch (even if they would batch if attached to a game object).

• Sometimes objects that on paper should batch don't for reasons unknown (or at least uncommented upon) to the list.

Will work:

• You can change UV mappings at runtime without breaking batching.

• You can change the vertex colors at runtime without breaking batching.

• You can change the mesh itself at runtime without breaking batching (assuming you stay under the vertex limit).

One best practice: look into texture atlases to reduce draw calls.