Actually, you can't really "install" the cross members after the fact, you have to install them as you build. The only way to get them in after the fact is to loosen up the top structure (not recommended!!!) so you could move the legs out far enough to go over the studs that screw into the legs. The only way to get them out is cut them and then remove the two halves, that's my method.
In any case, four of them are gone. I went under and I can't move the legs with all the weight on them, they're sitting in cups that have grippers into the carpet so they can't shift. That being the case, I can't imagine it being an issue without some of these supports. I don't see any issue in removing any of these that get in my way. As I work, a couple more may get sacrificed, the layout ain't going anywhere at this point! If, for some reason, there was a reason to add such bracing back in, I could just use some standard 2x4 lumber and run the screws all the way through the legs to install them. I'm not anticipating having to do this, but there's always a way to skin the cat if necessary. 
