the undergravel will pick up a lot of detritus and will get clogged up impeding flow. if you want to use it,you can but make sure you clean them weekly. also when you clean do one side at a time so as not to kill off all the beneficial bacterias.

as for your skimmer,not sure what you mean by no foam? if it's what i think it is,have you check your tank salinity? skimmers typically won't skim if salinity is very low as there will be no or low surface tension.