That substrate seems to be OK, I can't find any references to it leaching ammonia for months.
The first thing you need to find out is how hard is your water. The fish you mention need soft water so if you have hard water you'll need to rethink the fish. Your hardness should be somewhere on your water company's website, though some do make it tricky to find - if you can't find yours, tell us the name of the company and we'll see if we can find the page. When you have the hardness, please post the number here, and the unit of measurement as there are half a dozen units they could use - it's worse than inches/cm!
If your water is suitable, bear in mind that both the fish you mention need to be kept in large groups, so at least 10 of each.
The gravel cap - how large are the particles, and are they rough or smooth? Cories prefer sand so they can take a mouthful of it and sift through the sand eating bits of food caught in it, and then expel the sand through their gills. They can't do this with gravel. But if the gravel particles are fairly small and smooth there shouldn't be a problem; it's sharp gravel which can cut their mouths allowing infections to take hold. With gravel, the food falls between the particles and can rot there if too much food is fed to the fish, and this rotting food is the source of the infections.
So smooth gravel and not over feeding is fine; rough gravel and feeding way too much is not good. Fish need a lot less food than you'd think. Mammals like us use most of our food to maintain our body temperature while fish get their temperature from the water.
Then you need to cycle the tank. Since you plan on having plants, you could do a plant or silent cycle. With this method, the tank is heavily planted with fast growing plants, then we wait till we a certain they are growing and not about to die. The easiest way to do this is to take a photo of the newly planted tank so there's something to compare the plants to a couple of weeks later.
Once the plants are actively growing, the first batch of fish can be put in the tank, and it is sensible to test every day for ammonia and nitrite. If they stay at zero for a week, the next batch of fish can be added. If they read above zero, a water change needs to be done to get the levels down to zero.
@Matt should be able to help selecting plants suitable for your tanks size.