Very very few shops will tell you about fishless cycling. Mostly they are old school and say to just add fish, not bothering to tell you about the water changes or that they'll make more money when all your fish die. Or they will sell you bottled bacteria and omit to mention that most of them say to stock the tank slowly.
And it is all to common for shops to lie to make a sale. They should not have sold you all those fish for a 25 litre tank. Well, I suppose they could just be totally ignorant.....
You should clean the gravel. There will be fish poo and uneaten fish food in there decomposing to make even more ammonia. If you haven't cleaned it yet, do some of it at the next water change. The mess will have mounted up (unless you have been doing gravel cleans) and you won't get round all of it at one go. Once it has all had a deep clean, next time you'll be able to do the whole lot in one go.
But don't touch the filter unless the flow from it is visibly slowing down. If you do need to clean it, squeeze/swoosh the media in the water you take out during a water change.
I'm afraid just the neons makes the tank overstocked. Adding the guppies pushed you to 200% stocked according to the community creator. And it depends on what the catfish is as to how much that adds to your stocking. A corydoras or otocinclus won't add much but the problem with them is they are shoaling fish and you'd need at least 6 of them. If it's a plec of some sort, there are none suitable for a tank your size I'm afraid. And they are poo machines
To make this tank work, you could keep just the guppies. That would make it almost full. But you would need to rehome the neons and catfish. Or rehome the lot and get either 1 betta (siamese fighting fish) or 6 male endlers.
Oh, and I should ask, are the guppies males, females or a mixture? Guppies breed worse than rabbits which will only make the situation worse.