The generalities of the Astartes Unit Specialties are:
Scouts: Initiates that provide reconnaisance and support to the battle force with greater mobility due to lighter armor (ecept Space Wolve where their Scouts are actually Veteran Warriors that specialize in operating behind enemy lines)
Assault Marines: Excell in close quarters combat and can maximize this advantage with the use of Jump Packs expediting them into action
Tactical Marines: The core unit for most chapters, most well rounded at ranged and close quarter combat. Performs the generalist tasks on the battlefield but individuals can specialize based on experience and prowess in battle eventually leading to advancement to Sergeant or Veteran status and rank which could place them in a position of leadership
Devastator Marines: Heavy Weapons specialists
Apothecary: Unit level medic that can provide combat support. With higher skill levels can perform field surgery and is charged with the responsibility of retrieving and recovering a fallen Marine's geneseed
Librarians and Chaplains: Psykers and depending on Chapter and path choice can become quite an imposing force on the battlefield
Dreadnaughts: Venerable warriors mortally wounded on the battlefield then encased within an Adamantium Sarcophogus to extend the life of the great and experienced warrior so that his knowledge and prowess can continue to serve when his body is no longer able
Ranks and Positions of leadership can vary by Chapter and so Sergeants, Veterans and Captains and so on are chosen by the Chapter and elevated within the Deathwatch based of a multitude of criteria.
The short answer to your basic question is that most of the Marines that make up the Astartes are Tactical Marines. They are basically the generalist battlefield combat specialist. They can shoot well and manuver and when necessary engage in close quarters. Advancement to leadership positions can come more easily in one who knows the many ways a battle can flow but leadership isn't restricted to Tactical Marines only. Any Marine can eventually be placed in a position of leadership if they have the requisite qualities, regardless of their speciality.