Can one say
a. He isn't foolish to believe what they say.
b. He isn't an idiot to believe what they say.
c. He isn't smart to have made this discovery.
d. He isn't a genius to have made this discovery.
e. He isn't smart to write such songs.
f. He isn't a genius to write such song.
?
The idea is basically that : The fact that he has done .... doesn't mean he is ...
Many thanks.
I would say that grammatically your examples are fine, but from the perspective of how native speakers would say such a thing, you're a bit off. A native would probably phrase it like this:
He's just not smart enough to have made this discovery.
He's just not smart enough to write such beautiful songs.
But these would rather imply that you doubt the claim or you can't believe that he has done or does those things. To express the idea that he has done something does not mean he is such and such, you could say this:
Although he's not that smart, he was able to make this discovery.
Although he's not the most talented songwriter on earth, his songs are pretty good.
