There is no advantage to running 18/6 other than saving electricity/lamp life.
BUT the disadvantages are less NO3/PO4 consumption. Plants only photosynthesize when the light are on (lets ignore the Krebs cycle) so leaving them on 24/7 is best for nutrient uptake. Remember that photosynthesis involves the removal of CO2 (an acid) and the addition of O2(a base.) So whenever lights are on above any part of your tank you are leveling/raising pH.

Day cycle is for vegetative growth, Night cycle is for reproductive responce(circadian clock.)

Home Depot bulbs are fine for a fuge. They are low Kelvin/high intensity/cheap which are all good for macros.