I think they are like that. They aren't related to leveling, its skill levels. They said you can't get them all. So maybe you can get 10 by leveling a skill to 100, but there are 15 total talents in that skill. So you have to pick.
No you pick one each time you level up, yes many perks probably have level or skill requirements, you might also be required to have the previous on in the perk tree for the top level ones.
But you can select perks freely. if you level up because you made a lot of potions you can take a blade perks, you are not restricted to alchemy perks. Again it makes sense, that if you levelled up before your alchemy reached 75 who is required for poison mastery. Next level you raised alchemy one skill point to 75 and mostly used blades, on level up you get poison mastery.
This avoid the number crunching level planning from Oblivion.