You're probably a 42 in Buttero. Maybe even 41, depending on exactly how tight you want them to fit. I wear 43 in the Buttero sneakers, 44.5 in Vass F, 10.5 in Carmina, kind of between 10.5 and 11 on G&G.
Personally I like the Vanda examples better than that GG flower. The GG is a bit loofah for my taste. I have a couple and wear them occasionally. I think they are alright for anyone who doesn't take themselves too seriously.
When I think of leather and chinos, I think of either the LP/Cucinelli style or older Brando/Dean in a bomber or something. The more current streetwear-y leather jackets look much better with jeans to me. Maybe they only look good with jeans, and a specific kind of jeans. Not really sure about that 5-zip with the chinos.