When I transferred from my 55 to the 180 I did it all in one shot. I was able to move the 55 gallon to an adjacent wall and set up the 180 pre mix the water for a day or until it was all clear.
First thing I did was use extra tanks I had laying around to place rock, coral and fish seperate. I put the Rocks in and placed the coral as I was dripping the fish into the new tank. My stock list was 1 tomini tang, 1 clownfish, 3 chromis and 2 cleaner shrimp. They all made the transfer quite easily. As long as the parameters are approximate it should all be good.
I would wait a while to add any new fish beyond the ones you already have though.
Don't reuse the old sand only use the live rock.
