Can I use this on the Rocket Milano?

Yes, this will work on all Rocket machines, just make sure to select the E61 version.

What brush can I use to clean these?

We don't carry a cleaning brush specific to the mug. I recommend a sponge or dish brush.

Does the water from the holding tank travel through plastic or metal? Does the water ever touch plastic?

The water inside the machine travels through metal. It does touch the plastic in the brew basket.

How does the F4E Nano V2 compare to the Mignon Silenzio and Specialita? it seems the F4E has larger burrs (faster grinding than either Mignon?), digital programmable dosing (on par with Specialita and superior to Silenzio? but how do they all compare in terms of actual quality of the grind for an espresso focused grinder, noise during operation and quality of the parts used in the machine? Is the F4E serviceable or is it more financially feasible to replace it if it has, for example a motor or display screen go out on it? Thanks in advance.

The F4E is about the same speed as the Silenzio, the Specialita is the fastest of the three. The build quality is roughly the same on both. The Eureka's are the quietest home grinders we carry, the the Fiorenzato is not especially loud. Both brands are serviceable if there is an issue.