You may only have one Sergeant per squad.I don't get the concept of seargents at all. Do you have to have exactly one to be in your tactical squad? Can you name any troop your seargent? If you can't, the SM tactical squad sprue comes with enough materials for two seargents right? Thank for all of your input.
That Sergeant may be upgraded to a Veteran for a certain points cost (note that with Blood Angels and Dark Angels the Sergeants are automatically Veterans).
Whether Veteran or not, he may swap the Bolter for a close combat weapon and a bolt pistol. Alternatively, you can just leave him with the Bolter.
If you upgrade him to a Veteran, he may buy additional wargear from the armory - Power Weapons and Power Fist are the most common choices, but combi-weapons can certainly be worthwhile as well.
As with any squad member, the Sergeant cannot be picked out in close combat or shooting (excepting the torrent of fire rule).
You'll generally want to mark your Sergeant out somehow. Usually the weapon choices alone do this, but if you leave him with a bolter you'll want a different helmet, colour marking or something else to differentiate him.