The organizational structure of a Marine infantry company of that period was based on the mission; i.e.: amphibious assault and rapidly moving ground combat operations. A Marine sergeant would have been a squad leader in an infantry company, in charge of 8 or 10 Marines with a corporal as his assistant. A Marine infantry sergeant at Guadalcanal would most likely have been armed with the M1 Garand rifle (great efforts had been made to re-equip Marine units with the new M1's in anticipation of the Pacific offensive).
The squad BAR man would most likely have been a PFC or Corporal at most, with that assignment usually based upon proficiency with that weapon.
Neither M1 Carbines nor Thompson SMG's were standard issue weapons for Marine infantry units of that period. Those weapons were usually provided to support troops in other roles.
The 1903 Springfield rifles were used in some quantities, and were generally regarded more highly for accurate shooting by many Marines. For the most part, however, the '03's had been phased out of front-line combat units and replaced with M1's prior to the Guadalcanal operation, although sniper-scouts were equipped with sniper versions of the Springfield. Again, a sergeant was not likely to have been part of a sniper-scout team, which were usually privates and PFC's.
Other weapons likely to have been present would have included M1911 and M1911A-1 .45 caliber pistols (usually issued to officers and senior NCO's), and the H&R Reising model .45 caliber SMG (issued to quite a few radio-telephone operators and some others functioning in direct support of infantry operations). Each Marine company would have also had a weapons platoon, providing 60mm mortar and Browning .30 caliber machinegun capabilities.
A very few Marine units were armed with the Johnson .30 caliber semi-auto rifles, adopted as an alternative to the M1 Garand during the run-up to WW2 combat operations.
Rifle grenades were used in some quantity, with a squad member typically designated as the grenadier. The grenade-launching attachment for the rifle, and blank cartridges used to launch rifle grenades, generally prevented the grenadier from quickly changing his weapon back to standard use with live ammunition.
Hand grenades were in general use by Marine infantrymen. Flame throwers were in general use, although many Marines balked at going into combat with a large tank of gasoline strapped to their backs.
Each marine typically had a bayonet, and the fighting knife was in general use (either the USN Mk II style, later popularized as the Marine fighting knife, or the earlier PAL RH-36, but any number of private-purchase knives would have been seen in abundance).
Each Marine infantryman carried an entrenching tool, a small shovel used for preparing fighting positions, digging latrine holes, etc. The entrenching tool proved to be quite effective in hand-to-hand combat.
