Just as Gordy stated, there are rules governing operations such as you described. With that said, I've experienced considerable flexibility with the FAA's approval (or denial) of such congested area plans (assuming one was required and/ or submitted and approved). Approvals tend to vary greatly from one region to another and/ or one inspector or POI to another. Some regions were next to impossible to get an approval from while another region's inspector approved a virtually identical plan in less than 5 minutes telling me to; "...have fun and don't drop anything".
A good source of self educational information would be the FAA's
Flight Standards Information Management System, or FSIMS as they are generally referred. Try,
Flight Standards Information System (FSIMS) and enter "external load" in the "Quick Search" box at the top right. It should return several documents that cover several questions you've raised. You could also search the FAA's Advisory Circulars. There used to be a couple ACs that covered external load operations/ training/ congested area planning.
Good luck.