How to stop navmesh from overwriting one another

I have a giant issue that has brought my game to a halt...

I'm using Unity Pro (w/ Droid Pro) and I'm using Navmesh and Agents. I need to utilize the fix for agents of different sizes or radii. The only solution I found was this,

1.  Generate unity NavMesh for radius 10 (for big agents)
   

2.  Store navmesh as regular mesh on level, name it Mesh1. It is stored in a raw format with the folder with current scene. Or you may use any navmesh generator.
   

3.  Assign Mesh1 NavMeshLayer to a layer "BigOnes"
   

4.  Generate unity NavMesh for radius 1 (for small agents)
   

5.  Delete Mesh1 (or it could be done before release)
   

6.  Assign small agents NavMesh layers mask to go with layer "Default" (or whatever layer name you use for small ones)
   

7.  Assign big agents NavMesh layers mask to go with "BigOnes"
   
   

Firstly, the third step doesn't make any sense to me, nor anyone else who has replied to these answers and secondly, when I do this "fix", my BigOnes navmesh layer overwrites my default (and other water layer I've created). Ideally, I would like to have 3 layers. One default layer that ALL AI can traverse. Once water layer that only swimming AI can traverse (so far this all works). And then one "bigOnes" layer that only giant enemies can traverse (this doesn't work). This should be very easy to implement, but when I do, my blue default layer and purple water layer are replaced with the "bigOnes" layer. Any ideas or fixes? Any help would be EXTREMELY appreciated!!! Thanks!

~Alex