I have never personally had a group of yellow tangs, but have seen it done. It seems to me that tangs behave differently depending on the size of the tank. Yellow tangs can be mean sobs in a smaller tank, but it seems when they have more room they are more mellow. In my 240, I seem to have no aggression issues with any of my tangs, the exception being a Sohal that I had that was just plain evil. He would slash the heck out of another fish for no good reason and swim away as if nothing had happened. Sneaky, evil fish. So all that being said, I will go back to an earlier post I made, and say that tangs need space. The more tangs you have, the less personal space they have and the higher likelihood you will have a problem. And, you have to account for their future size. When I got my emps and my nasos they were much smaller. they have turned into very large fish now. I watch them daily to see if any issues are beginning to arise. So far so good, but I remain wary. You need to have an exit strategy if what you think will work out, doesn't.
So as an answer to what you specifically asked, I think 5 yellow tangs, a hippo, a kole and a blonde naso are a lot of tangs. Maybe it will work with an open landscape, but are you going to put other fish in there too? I think you are better off with max 3-4 tangs.
I think I wrote poorly when I said that most people with a lot of experience with Nasos sold them. What I meant was that people that have had a lot of experience with Nasos are generally importers and fish stores. They see tons of Nasos. But they don't keep them long because they are in business to sell them. So, they have lots of experience with Nasos, but not much keeping them long term. I have kept them long term, but not very many times. So my point is, is there an "expert" out there that has kept them long term, in lots of different circumstances, such as different tank sizes, different tank mates, different layouts, so that they can definitively say that this practice is bad and this practice is good. I suspect there is no one out there with that kind of experience. So with that being said, all I can do is offer my limited experiences and people can decide what to do on their own. I watch my fish closely and react based on what I see. Sometimes I have to remove a fish. I have removed an Asfur Angel and a French Angel from my tank because they were affecting my Moorish Idol either with aggression or simply out competing it for food. I removed my Sohal because it was a killer. It is difficult to get all fish to get along, just like it is difficult to get people to get along. So I 'd say take into account a fish's tendencies, and decide whether you want to try, then keep an eye out and make sure it is working or not.
Oh, and go Flyers!