Talentwise, Gallo seems better than Hari. And Hari was one of the best (if not the best) players on Leksand last year, and he will probably keep on being that in the SHL.
Regarding Kozma, I'm not sure if he is in that high regard as of now. He wasn't chosen to play for the U16 squad when they went to Canada (that could have been his club's decision though, but there were other players from foreign clubs who joined the team). He is putting up decent numbers way above his age level, but it's still Division 2/3, which is not ideal for development.
He is probably ineligible due to the Hari-rule (he needs two consecutive years in Hungary after the age of 12, he only has one as he was still 12 when he went to Sweden), so I doubt he will play for the NT. I do not recall his name on rosters of Hungarian NTs but even if he has played for one, it's not official until it's an U18 WC, for which he is almost certainly will not be eligible.
Watch out for Marcell Revesz though. He is playing well in Switzerland and was a member of a pretty good U18 NT this year despite being just 15. We have had a few good prospects in Switzerland in the past, but Kristof Hentes gave up hockey for his studies and Zsombor Kiss is coming along nicely, maybe we will see him next year in Kiev.