Yeah, definitely cover the intake if you want a filter other than the sponge filter. You could cut a piece of sponge to fit over the intake. You can also use a fish net or a ladies nylon sock (the free ones you get for trying on high heels haha). I breed guppies in my 20 long, I have a cheapo azoo sponge filter with a tetra whisper 10 air pump. Very quiet and has been running for 2+ years with no issues. I also have the tetra whisper 20i internal filter which I bought with the tank's rebate thing, with an old fish net attached. It's such a quiet filter and very durable, but I lost so many fry to that thing without the net. When I bred mollies for a few years that was the #1 cause of death except bob the betta, LOL.
I breed as feeders right now and grow them to about .5-1". Not including the parents haha. There is about 50 babies I would guess, plus my 8 corries. Mine are cannibalistic though for the newborns or week olds for whatever reason so grow outs are kinda rare hahahah. You should have a reasonably strong filter and keep up on water changes. I try to do at least 3 25-50% changes a week. The fry are usually more sensitive than the adults to nitrates and the like.
Also for plants, floaters are good. Guppies like to stay at the top and they will eat microorganisms and algae from them. The fry feel very safe in the leaves and roots as well. In my tank right now I have hornwort, anacharis, and stem plants. I have hygrophila, jungle and corkscrew vallisneria, bacopa monnieri, and brown crypt wendettis. I use no co2 on this tank, they are all clippings or plantlets from my 90, which is fully flourish dosed and co2'd. Ferts in the 20 are from the fishies. I also have pothos growing out of the back. Lighting is a single t8 which is old, needs a replacement soon.
Oh, frogbit and duckweed is also good but that stuff will need maintinance or it will block light for your other plants and take over everything FAST. Hope this helps. Been keeping livebearers and undemanding plants for eons.