Also, remember that this is merely the standard OOB.
The Section in the film Aliens consists of veterans in the field, and operational units tend to deviate from the OOB the longer they're out there.
The fact thet they're just one section as opposed to two is already telling. There's no way to discern whether or not the second section is still in hybernation, but that would be a weird decision, that could only be explained logistically (low supplies/air maybe? Or more economical perhaps; "no wake up, no pay"?). At any rate; I can't recall any mention of the second section in the entire film.
But the fact remains that they're not just parade ground troops; they have loads of individual touches, such as non-regulation pieces of equipment and clothing, painted on slogans and symbols, etc. So additional equipment such as flamers or shotguns would be entirely possible (in the field)...