I currently have an 80 long, which is essentially a 6ft long version of your tank, and I keep 2 large male angels with rainbows, scissor tail rasboras, cories, a couple ottos and a lone GBR. They show no aggression towards anything in the tank but each other at times. They ignore everything else. They are both males and at times like to spar with each other. Never anything more than flarring and a shove or two and then they drop it. I have had angels in other set ups for years and the biggest mistake I made was adding new angels to established social groups.
I would not do 6-8, but I've seen 5 in a 55 while they are small work, however, pairing up may result in problems. If you do multiples, make sure to get them at the same time and the same size, groups do not tend to except new members easily. Your main problems will be pairs that want to spawn and dominant males in the same group. It will depend on the personality of the group you select. As they grow, you may have to take a few out. I find it easier to remove from a group than add to it if you think you don't have enough, but I also hate rehoming or trading in fish. Some will say it is ok to add females, but it can be very difficult to sex juveniles.
In a 55 I'd personally choose one angel that I really like and surround it with other species, but you could possibly do a few more.
