Hello and welcome to the forum! Your tanks look really nice.
Good on you for rescuing these fish. As you suspected, your tanks are far too small for the fish you have, and you are going to have some problems. The angelfish is going to very quickly outgrow a 57 liter tank. Some people do keep them in small tanks like that, but they aren't very happy (as you said, they're very smart and need room to explore), become stunted, and usually don't live very long. Do you know what kind of pleco you have? Common plecos get HUGE, but even the smaller species will be very cramped in a 57l. Similarly, a gold gourami is too big for a 35 liter tank. And yes, tetras and cories really should be in groups of at least five; ten or twenty is better, but you don't have room to add more.
So, you have some difficult choices to make, I'm afraid. The angelfish, gourami, and pleco are going to need bigger homes. The gourami should have at least a 100l (though if he is permanently stunted from his past life, he might be OK with a bit less); the angelfish 150 or so. Common plecos are giants, and will outgrow a 100 gallon tank, eventually, so hopefully you have something else!
If getting bigger tanks is not an option, it might be a mercy to find them new homes with someone who has a larger tank (check with your local fish store, if you have a good one). Then you could add a few more tetras and cories to the 57l (though that is still a very small tank for these fish), and use the 35 for a betta, some shrimp, or a nice school of dwarf rasboras.
It looks like you are doing a lot of things right, and these are very common difficulties, especially with rescue fish. Let's keep talking about it and try to find the best solutions.