[@Skwint][@Ollumhammersong]
If you don't pick the sergeant, there will be a NPC one at least in the beginning. 

Your overall objective is to retake Hive Primus from the rebels with an initial combat drop to secure a beachhead for the rest of the regiment and any other Imperial forces that may be joining the battle. From then on it'll likely be a nice urban combat slog, of course, you can always requisition new gear as situations and mission change.