Having owned 2 biocubes and 2 of the oceanic skimmers myself i would recommend opting in for a better skimmer. The oceanic skimmer is air driven and with a bigger air pump i had marginal success pulling up skimate. Im not familiar with the exact dimensions of the back chambers of the JBJ but i know the make some nano skimmers that will fit chamber 2 of the biocube (the tunze 9002 and the aquaticlife 115).
I do waterchanges on my biocube every 2 weeks (becuase i just use high quality salt and dont dose) but if you bioload is low and filtration is good monthly WCs are more than adequate.
Im not sure what LEDs your referring to but if you plan on sticking to softies you wouldnt need much light anyway... I am running 4 12" ecoxotic stunners on mine and the corals are doing well, but i also suplement by running the original lights on pure actinic.