Seachem Prime is often recommended for fish-in cycles as it helps to 'neutralise' both ammonia and nitrite. The effect doesn't last long though so you can't just add a dose and not do water changes for days. It does mess with the ammonia tester for the first 24 hours though so the reading for that one won't be accurate for a day after adding Prime. It is also very concentrated so a bottle goes a long way for your money - and you will need to use a lot of dechlorinator with all the water changes that go with a fish-in cycle. There are other dechlorinators that 'neutralise' ammonia, though they don't do anthing for nitrite.
I would clean the gravel once a week. Or a small portion every water change while you need to them daily, that way you'll do the whole lot once in a week. There will be fish poo and any uneaten food sitting in there, decomposing to make ammonia which will be converted to nitrite by the ammonia eaters, making your nitrite go up faster. Yes, there will be bacteria growing on the gravel and on every other surface in the tank, but the majority will be in the filter.
Personally, I think that the way some people go on about not disturbing the gravel dates back to the first type of biological filter. Back decades ago, it wasn't understood about bacteria removing ammonia and nitrite. When it was, the first filters to use bacteria were undergravel filters. These used a slotted plastic plate under the gravel, and a pump sucked the water through the gravel, under the plate to an uplift tube, up the tube and back into the tank. The gravel was the filter medium, that's where the bacteria grew. So disturbing the gravel with this kind of filter was not a good idea. We may have internal and external filters now but once an idea gets fixed in the public conciousness it is hard to get rid of it.
Feeding the fish is something tricky to estimate. For dried food like flakes and pellets, a very rough guide is the amount of food equal to one eye per fish. A fish's stomach is about the same size as its eye, that's the reasoning. (Yes, there are fish it doesn't apply to, but the guide is still a good one for beginners
) Most flake products contain everything a fish needs for its staple diet, but you can use a couple of different brands, or pellet food to vary it a bit. You can also feed live food and vegetables.
Live food is things like bloodworm, brine shrimp and daphnia. You can get bags of these from a shop, but they have to be used quite quickly and that would mean over feeding or throwing a lot away. And they have been known to bring in disease. The better way is to buy frozen cubes of them from the fish shop. You can chop off as much as the fish need and thaw it in a bit of tank water. They also don't have the same disease problem that alive live food does. One word of warning though - the cubes contain a
lot more than you'd think so start with just a small corner. Feed these once a week as a treat as there isn't much food value.
Platies and guppies do like veg in their diet. The usual vegetable to try is peas. Cook some, or save some from your meal, squeeze the skins off and chop up the insides till it's small enough to fit in the fishes' mouths. Make sure there are no large pieces - there is a thread by Jesnon in the Emergency Room on what can happen if a fish tries to eat a piece that's too big. Remove any uneaten peas or they'll go off - just suck them out with the siphon tube.