For a job that size, I’d probably use the excavators and dozers for the main work and keep the tracked skid steer for cleanup and handling the smaller material.
Since the farmer wants the roots removed, I wouldn’t rely on a mulcher alone. It might make quick work of the brush and small trees, but it still leaves the root systems in the ground. An excavator with a rake or root grapple would probably be the most effective for pulling the larger material while disturbing less soil than trying to push everything over with a dozer.
For the 1–3 inch stuff, a tracked skid steer with a grapple rake could be useful for gathering and sorting what’s already been pulled. I’d probably clear the heavier sections first with the excavator, then use the skid steer to clean up the smaller brush and roots.
I was recently looking into different approaches to vegetation clearing and found some useful information on Infrastructural guide on vegetation clearing. It also mentions using specialized equipment such as forestry mulchers and other machinery depending on the terrain and type of vegetation, which helped me think about separating the cutting/clearing work from the actual root removal.
