Keep feeding and doing water changes until the fry are large enough to move to their own tank. At approx. 4-6 weeks old ( depending on size), you can start feeding the fry a growth type food ( there are many on the market) that has been crushed up to make the pieces smaller. At that point, it's regular maintenance until the fry are ready to be rehomed.
Regarding the sponge filter, it's just an air driven filter that pulls water through the sponge to filter out the larger pieces of detritus and create a home for the nitrifying bacteria to live upon. What makes them safer for fish fry is that they are air driven so the suction power is not usually high enough to suck in a fry and kill it opposed to mechanical style filters which can.
As for plants, I am not all that familiar with what's available today to really answer. Better to ask some of the others who work with planted tanks.