Potential actions taken by a veteran commander (please note I'm not a veteran so I'm taking guesses and while I'm providing a possible scenario here probably full of holes I'm neither advocating for or against this scenario but simply offering it up as a possible difference).
1. The commander would have read Ripley's report on disk and would have known the contents in depth. Would have also spoken to Ripley at length if nothing else but to determine her trustworthiness. Would have possibly even read details of the inquest. (Gorman may have even done all of this).
2. Once getting to the colony, would have ruled out before even touching the ground the "downed transmitter". Would have secured the area, perhaps in a different manner then what was done - not sending in all marines at once but keeping a few in the APC to act as backup. Would have very quickly realised that something very bad had happened and Ripley's information is looking more and more correct. Again - something
3. Would use the PDT's to find them as shown, but would have (if not already before arriving - in fact possible areas of concern - like not firing weapons to damage the structure should have possibly been brought up before even leaving the sulaco) have studied the floor plans of the processor and would have devised concerns about firing weapons in there.
4. Two marines (Wiezberski & Crowe) would have stayed with the APC while the rest would have gone down to the entrance to the nest "I can't make that out to well what is it Hudson?". At this point, Ripley's testimony is to be believed and the dangers of the creature/s they're dealing with becomes very real.
5. Dropship is hovering staying on station ready to be called in.
5. Four Marines (Apone, Drake, Hudson & Dietrich) enter the nest along with Bishop who is armed with a flamethrower, Three marines (Vasquez, Hicks, Frost) remain at the entrance to the nest. This way escape route is left open.
6. Bishop takes point at critical junctures. If it is only at this point that the issue with firing the weapons in there is found out, then bishop is armed with a flamethrower at this point and is required to move forward. At seeing the colonists, hung up on the walls and obviously dead an assessment is made by the commander at that point, knowing that weapons can't be fired in there and knowing that the creatures in Ripley's report are real. Also, given that the entire colony has been overrun, and having seen the evidence of the "last stand" etc, it would be prudent to think that the enemy you face is a damned handful. The colonists would have only had limited weapons and the marines are now almost on the same footing, limited weapons, and in the heart of the nest. The assessment made by the commander is to either bug out right then to make a report to back home or to send bishop further in.
7. If sending Bishop in is the assessment, the marines in the nest stay put and wait for bishop's call. Bishop goes as far as he can and either encounters aliens or he doesn't. If encounter is made, Marines in the nest are under orders to retreat immediately. So Marines in the nest retreat back to the entrance of the hive and the other marines who were guarding their escape, then together they all retreat back up to the APC.
8. APC leaves processor and gets picked up by Dropship.
Secondary scenario also involves the APC going down to the bottom floor where Ripley took it to get the marines out and have the marines load out from there.
The whole point is that an experienced veteran commander would have recognised the threat and employed tactics to compensate.