i would return a lot of that stuff. as dave said, the skimmer is too small for your system. they don't work that well anyway. i would get one rated for 125-150 gallon tank actually, considering the sump volume. the brand i would probably go with would be octopus.
i too, would return the hydrometer and purchase a refractometer.
and thirdly, i also would use 2 heaters.
i would return the cannister and build a sump out of a standard tank. probably a 20 long. i know the space is limited under a 55.
if you are planning on keeping photosynthetic livestock, i would return the hagen t-5 lamps and purchase ATI lamps for that fixture. i see 2 lamps. is that a 2 bulb fixture? will this be a reef or a FOWLR