I don't use hoola hoops but I get the concept behind it. I like the idea of it for when older kids are wanting to play and the little kids are "bugging" them. I don't have any older kids right now but it would have come in handy when I had two school aged boys over a summer and they wanted to build towers and such but the little ones would always knock it down! I also only allowed the little lego out when the little ones were napping but the hoola hoops might have worked for that too! My kitchen has a hallway that leads to the playroom so I used to just put up the baby gate and the older kids would play in the hallway so that I could still see both groups but the babies weren't interfering with the big kids play.
I would say to start teaching the rules of the hoop as soon as possible. The earlier they learn, the better and the more success you would have at using them. Good luck!