What tubing material is that, it REALLY takes pictures bright like stainless.

Here is the typical deal.. depending on what rule set your building to they have minimum and maximum Plate sizes I would assume 6"sq is a minimum recommendation so going bigger is not an issue.. feel free to bolt through the plate it will not harm a thing.. typically when I build a cage I try to move all braces as close to the corner as possible so I can tie a box or plate into two surfaces. AT LEAST two in the case of a main hoop boxes. the closer to a corner seam or "subframe" rail the stronger you'll be.