You should be fine. 12 inch partitions in a standard 48x18 55gallon will have 21.6 gallons worth of room left in it.
I run a drilled 210 w/ 55 gal drilled sump - 30 total gallons in sump w/ external drive and no check valves - last stage is a 12" wall w/ auto drop float and I've had several blackouts w/ no overflowing water. Only thing I can add is you'd be suprised how much phosphate only a few fish can produce in even large bodies of water. I don't think phospates are as important w/ deepwater and softies as they are w/ stoney corals but sometype of phosphate remover will probably be needed w/ the size of your stock esp. with the several clams you plan. Also be-aware Dwarf angels might nip the mantels of clams ultimately killing them.
Only thing i would change would be personally i would rather a blonde naso over the 3 yellow tangs. and i would want a male blue throat trigger in there.
sounds good...take a ton of pictures....especially when you are building your 55 gallon...because i also have a 220 that i just resealed a few days ago, and have been pondering on how to do my filtration...but the stock sounds good..
anemones in mind??
and yeah the tank/stand looks sick...
can't wait to see the finished product...