I would set up one of your 55 gallon tanks for a temporary tank. Fill it about 25% - 35% full with fresh saltwater. Add the heater and a couple power heads. Have a spare tank, tub or tote of saltwater made up and heated.
When moving, I would put the fish in a cooler with a battery powered air pump. Skip the bagging. Save enough of the old water to fill your 55 gallon the rest of the way. Use totes or clean garbage cans to keep rock covered in water.
Warm the room up to 78 degrees if you can. This may help reduce heat loss while you are getting everything ready. Add some of the cleanest rock you have to your 55 gallon tank (about 50 pounds if you can). Try to swish the rock in the tote to remove debris before adding to the 55. Then fill the 55 gallon with with the old water.
Now you can start to acclimate the fish. I like the drip method but others may disagree. If you drip acclimate this is where you will need the extra tote of saltwater mentioned above. As the 55 drips into the cooler you will need to refill the 55 with new saltwater. This process can take a couple hours.
Once the fish are acclimated you can move them to the 55. I would not run a long light schedule on the 55. You don't want to give the algae a chance to start again. I would do a 10-20% water chance every week on the 55 until you move everything to the 210.
Make sure you keep a heater and powerhead in the container with the rock.
Now you can start the process of cleaning the 210 and all the accessories that went with it. It looks like there are a few softie corals on the rock. You may want to try and save them.
Once it is clean I would fill it about 75% with clean salt water. Add sand(if you want it), heater, wet/dry filter (if you are going to use it), skimmer and a couple more power heads. It is very important to keep the flow moving across the live rock. That is your primary filter.
Clean the live rock by swishing and scrubbing with a brush. Make sure the brush is clean and has never been used. Give the rock a final rinse in fresh saltwater before adding to the 210. After the rock is added fill the 210 the rest of the way with fresh saltwater.
This may be some what controversial. I would keep the 210 dark for a few weeks. Even cover the tank if you can. The tank is going to cycle and if you leave the lights off it should kill any left over algae. Make sure the powerheads, heater and filters stay running.
Once the tank is ready you can start adding your fish slowly. Maybe one every couple weeks. Add the tang last. Start adding the lights a few hours a day. Building up to your lighting schedule.
As you add fish add a little of the rock from the 55 to the 210. After the last fish is moved you can move the last of the rock to the 210.
Drain the 55 and clean. By then you will be wanting to set it up as a reef tank.
Go outside and sit on the swing and enjoy the nice warm day. Because by this time it will be spring.
If you have an RO filter use it. If not go one or buy some RO or distilled water from the store. One of the keys to success is a good RO/DI filter (in my opinion). Don't use it for your fresh water fish just the saltwater. You can use tap water but you may end up fighting phosphates, algae and bacteria.
As the tank evaporates you will need to top it off with fresh water. Use RO for this also.
Kind of long and I still missed a LOT of steps. I hope others step in and fill the holes and offer other opinions.