I personally have the same gravel you have mixed of the bigger with smaller ....and i love it. Not sure may be it different because i do only 100% water changes though. But i though it also easier to siphon the gravel like that?
You need countless generations for such effects to take place. Even in fish. And given the fact that most are bred in captivity and released in aquariums which have a 50-50 chance on what to have in the bottom where they breed again, i believe the possibility is quite remote. Its like saying that bettas have become accustomed to their fins been torn from plastic and they can resist now because they are bred in this conditions.
I dont say throw them in the hardest environment. I am just saying that fine gravel is not going to be so much of an issue. Unless gravel for someone means rocks that is.
No it doesn't need to take countless generations for "such effects". And even if it did fish produce at a steady rate that it would happen in a short amount of time.
You should be fine with the setup. I'm curious to see what the tank looks like during the day & during night when it's illuminated.
Cories are crazy like that and sometimes have the tendency to headbutt things. Have you noticed them swimming to the surface to breath air?
I don't think you're overstocked at all.
What type of filtration system are you using? I know 40G don't come with starter kits.
hopefully this link works. BTW, despite having that betta hammock, Baloo normally sleeps on the gravel towards the back left side. This morning, I found him sleeping on top of that blue plastic plant. It was pretty cool to see him wedge himself in there.