Individual bettas are unique, so you should keep an eye on him on the first days anyway. You never know.
Typically, a baby will not be aggressive, I have seen many very small bettas kept in community tanks with a lot of different fish.
Additionally, if you add the betta while other fish are already present before, the betta will not feel that it's HIS territory and that he should defend it, so that will help too.
And because he will grow up with them, as you said, he might stay peaceful.
However, it's very possible that, growing up, becoming bigger, he will one day become aggressive and attack the other fishes and dominate them, especially colorful ones. So to answer your question, they are aggressive by nature, they don't need to be taught that. They see another fish that looks like them and they want to prove that they are bigger and stronger.
Your setup and idea sound very safe. How big is the tank?
Edit: Be aware that bettas can also be the victims of aggression. They are slow moving and have long, flowing, tasty-looking fins. Make sure the baby betta is not being bullied by faster fish.
According to my experience: The snails will be no problem, but the shrimps may be eaten by the betta eventually.
I'm pretty sure you will have to separate the betta from the endlers and shrimps when he will be bigger, so prepare for that.