not sure if I missed any, but anyways first tht will need to go is the featherfin cat, grows too big too fast. Once he's gone you'll have time to upgrade to a bigger tank or exchange some of your larger fish for smaller ones. Temperature is fine couple of degrees up or down won't be an issue and neither will be feeding you can feed them 2-3 times daily a little bit or once but more or once every 2 days more it's up to u. Good luck and keep us posted
Welcome to MFK! You came to the right place for advice, and I think your doing a great job so far too. If it were me I"d get a bigger tank and filter if you like the fish. I recommend a 55 gallon minimum. If you don't have the moola right now I would do trade-ins with your local fish store (call around to find out who will accept them) or post on Craigslist to help you rehome the synodontis and gouramis. But you never know you might get lucky if you post an ad on Craigslist asking for free tank/filter/etc. Lots of people have aquarium equipment lying around gathering dust.
Feeding - With a tank that small I recommend once a day a small pinch each of flakes/pellets. If you upgrade to a 55 or larger you can do twice a day.
Water Conditioner - I would get Prime and follow the directions. You can get it at Petsmart if you have a store nearby. The large bottle should even be on sale this week.
Water Changes - Once a week I would do a 50% water change myself. If there is gravel be sure to get the vacuum down into it to suck out the detritus that accumulates underneath. Consider switching to sand. It's alot cleaner. You only have to hover over the sand with the vacuum to get the fish poop and stir it up once a week to release any trapped gas. Petco has small bags of sand that are relatively cheap in the USA.
Temperature - 26-28 C is fine.
I have a tropical community tank too. Love them! Good luck with your tank and fish.