That's not how marines were organized during the Great Crusade/Heresy. Legions at that point were front-line combat units, armies in their own right. A tactical squad in the heresy is just marines with bolters. A dedicated support squad would have meltas/plasma.

After the Second Founding, when the legions were split into chapters and banned from commanding non-astartes forces, space marines transitioned into a special forces/support role for conventional imperial forces. That's why they started using mixed loadouts. They couldn't rely on having a whole seperate squad of guys to do one specific job. They had to be able to deal with all kinds of situations in vastly smaller numbers.

Some units break that mold, but those are only certain veteran/specialized units.