SClean4

I was wanting to put some small center caps on my wheels to give the center a brighter/better/fuller look, not the large center caps that cover the lug nuts, etc. but this may also be an option later depending on the outcome of this. I also plan to get shiny, chrome-covered lug nuts (stock lug nuts currently). My questions are:

1) What is the name of the part in the middle of the hub/rotor that centers the wheel and looks like a small bowl? Also, there is another part that appears to be screwed into the "bowl." I've searched and have found that it might be called a Barrel Coupling but I'm not really sure.

2) Can I add center caps? I tried to measure and I'm guessing it's somewhere around 60-62 mm but a part that's screwed into the "bowl"/middle is sticking/popping out a little from where the center cap surface would be on the rears... maybe the center cap will or can stick out if I can get certain kinds, don't know. But the "bowl" might also be in the way of center caps, too.

3) Why do the rears have the part screwed into the "bowl" part but the fronts have nothing screwed into it? Is this the preferred setup and do the rears need this part in the bowl or can it be unscrewed, etc.? I haven't been able to move/unscrew it yet.