I'm recently making my first "highpoly" experience and i ran into a small problem: when i subdivide the model, all edges become smooth the same grade (any suggestion for better grammar?) but i dont want the hard edges to become smoothened, is there a way to achive that without having to manually subdivide everything or adding useless divisions to the hard edges (before subdividing) in order to make them less smooth?

To make edges have a nice smooth bevel, and retain volume you need to add control edges, control edges are extra edge loops running parallel to the main edge you need to retain shape. These edges provide more structure for the Sub Divide Operation to work with, and prevent the edge from collapsing.

Local Sub Division Refinement, can become very messy if used too much, on too many different parts, and at different stages of the mesh. Its better to create the base mesh in one go, add the control edges, then run the sub-divide mesh operation.

If adding in control edges becomes too difficult and/or the mesh requires too many in one small area, its better to breakup the object and use whats referred to as "floating geometry" for additional details. Usually for a Hi-poly its best to make it in the same fashion as it exists in real life, using the same parts and such, instead of a singular mesh. So bolts, screws, and other things would be floating geo, a gun would have the upper a separate piece from the reciever and stock and so forth.

For your RC helmet, I suggest breaking it down into parts, like the face, back, crown/top and the side bars with the "circular section" all being separate. Anything like breathing grills, vents, etc can be floating geo.

IF the Low-Poly you bake the model down too has substantially less parts, then make the UV map reflect this and only map the parts that will show up on the low poly and move the rest that won't be used in the final off to the side.