If you have the tank well planted you won't need to do a fishless cycle using ammonia. In fact the ammonia may well kill some plants.
Plants prefer ammonia as their source of nitrogen. They take it up faster than bacteria so with a well planted tank the plants can remove all the ammonia made by the fish. And plants don't turn the ammonia into nitrite, they use it to grow.
With well planted tanks all you need to do is make sure the plants are growing well - you suggest waiting a couple of months for this - then add fish a few at a time, monitoring for ammonia and nitrite daily afterwards. If both stay at zero for a week, get the next batch of fish. With 240 litres water it should be safe to add a whole shoal at one go.
As for the substrate, there is some debate as to whether plant substrates are much use because they come with fertiliser in them which gets totally depleted in about a year so you then have to feed the plants as though you didn't use a plant substrate. And some plant substrates release ammonia so you have to wait for that to drop to zero before you can get fish.
Fish that like to dig will also bring the plant substrate to the surface. Cories shouldn't dig too much but I have no idea about porthole catfish, I'm afraid.
You need smooth gravel with cories; sharp gravel damages their barbels and allows infections to take hold. Do you particularly want a black substrate? I've read a lot of comments that most black substrates do not look black when under water, and that fish poo etc is very visible on black substrates. There is also the risk that black, along with dayglo pink and blue, could be dyed or coated with a coloured layer.
This may sound that I'm trying to put you off planted substrates and black gravel/sand. I am

Bleeding heart tetras sound much better. But in that sized tank I'd get more than 8 to 10

Seriously Fish doesn't caution that they are fin nippers so they should be OK with pearl gouramis.
Just to warn you - I have never seen dwarf flag cichlids in a shop (Laetacara curviceps). But I have seen, and in fact once owned, Nannacara anomala, the dwarf golden cichlid, which looks quite similar.
I have not come across pothole
cichlids catfish before, but according to Seriously Fish they are very peaceful and won't harm even very small tank mates except perhaps fry. SF suggests either one, or 3 to 6.
One bristlenose is fine. And you already plan to have wood in the tank.
Would you want other shoaling species? There are so many to choose from with soft water
