We have a difference in opinion. I believe that the 1 lb per gallon rule on L/R is overstated. If you have a good skimmer, and good filtration , that much L/R is not needed. When I first started in the hobby, everyone said put as much L/R as you can. While this is good, there are trade offs. Even if you put too much rock in a 6' tank the tang will not have enough room to roam.After seeing so many long term success with less rock and good filtration, no longer a strong believer in all that rock....also recently read that many are trying to ban even the collection of broken pieces of L/R from the ocean as they are now realizing that those same broken pieces have actually aided in the extension of the reefs over many years. Aquacultured rock s/b used and even that only as much as we need.
As far as YT in 55g, I still think it's ok although a 6' would obviously be better. I have seen too many long term success stories of thriving YT's in a 55g. Again it's always preferrable to go bigger.....8, 10, 0r even 12' for that matter.