Your tank is the same as mine. I found the hardest thing with the 12 in wide is trying to aqua scape it, we started it with 20 lbs of live rock as well, it wasn't enough, I got it to 50-60 lbs, but there wasn't a lot of room. I had an in tank skimmer and 2 heaters as well. i've been redoing this tank and making different choices with the rock and equipment. You are definitely going to need more lr with the fish load you are planning. Be careful with the tangs, I had a yellow and he got aggressive very quickly, had to sell, catching fish means removing most if not all your rock. Just go slow.....my 55 finally settled around the 9 month point, but I made every mistake possible when starting.
Make sure your fish choices work with your overall goal for the tank.
Are you running a sump? You can always add rock to the sump to make room in your dt. Also a great place for equipment. I don't have one on mine but would make things a lot easier!!
Be careful with that going bigger thing....I went to a 180 at the beginning of the year, once the addiction gets ya, there's no stopping it!
