In my fantasy world a scholar magician creates a sub-specie of warriors with an optimal natural armor against medieval technology weapons. What would it be like? How can I make it as flexible as a XVII century armor? What is the minimun it could hinder mobility and resistance?
Hair as hard as a horses mane.
The Gauls are tall of body, with rippling muscles, and white of skin, and their hair is blond, and not only naturally so, but they also make it their practice by artificial means to increase the distinguishing colour which nature has given it. 2 For they are always washing their hair in lime-water, and they pull it back from the forehead to the top of the head and back to the nape of the neck, with the result that their appearance is like that of Satyrs and Pans, since the treatment of their hair makes it so heavy and coarse that it differs in no respect from the mane of horses.
Your warriors have the added benefit that their entire body grows a thick coat of Sasquatch-like hair. Like hair on the head, this body hair continues to grow and grow, achieving great lengths. The warriors lime and twist the hair into hardened spikes, then artfully weave it into armorlike braids which cover their entire bodies (except for uncovered orifices to allow perception and bodily functions). Before a battle these braided warriors affix shells, beads, charms and bits of metal to their natural coats; these offer additional protection, add to their fearsome appearance, and make a terrifying rattle.
Bonus: if they want to pass as regular humans it will be an hour or two of shaving and they will be good to go!
Not limited merely to our own science
You've already stated that this world contains scholar-magicians who are capable of creating new species of person. That means that magic is totally a thing, and people can wield it. By extension, if you want to make the perfect warrior-people, your best bet is to have them be natural, instinctive magic-wielders, who reinforce their own skin, muscles, limbs and organs with magical power.
Beyond that, you're creating a warrior-race. They're not going to charge into battle naked. That would be silly. The question shouldn't be "how well-armored can we make your skin", but "how hard can we make it to kill you overall". Start out with harder, denser muscle, and thicker, harder, heavier bones. That'll give additional strength as well, which is always good. It'll also increase the food intake, but that's okay. Make the skin thick and leathery, with a protective layer of subcutaneous fat under pretty much all of it. That might interfere a bit with the fine motor control, but it shouldn't be a problem for melee combat, and it'll provide a nice bit of cushion against impact of all sorts. Reinforce the skull in particular.
There's limits to what you can do against piercing weapons without armor, but if you decide that you are concerned about getting attacked without your armor, then adding a layer of keratinous and/or boney growths over much/all of the skin will help, effectively serving as studs.
Take that combo, add a few more tweaks (expand and reinforce the ribs, better protection for the neck, reinforce the skull) and you'll have someone who's at least quite a lot harder to kill. Throw some passive magical reinforcement on top of that, and I imagine you'd do quite well for yourself. They'll need at least twice as much food as a regular soldier, and they'll suffer badly if it gets too hot, but in cold enough climates, when well-fed, they'll be monsters.