To answer your question for the following example you gave...
command stand (1)
rifle support w/Boys (1)
rifle/lmg squad (3)
rifle squad w/2'' mortar (1)
The numbers in RED above are the number of stands of each type required. Those number correlate to the numbers in RED in my explanation below (example 1 x …), thus you would make...
1 x Command stand (2-4 x figures at least 1 of which is a command figure)
1 x Support stand with a Boys ATR (1 x figure and 1 x ATR figure)
3 x Rifle/LMG stands (3 x figures and 1 x LMG figure per stand)
1 x Rifle squad w/2" mortar (3 x figures and 1 x 2"mortar figure) .
As to the Rule you are mentioning...
Infantry Drop-In, Page 11.2, 1st paragraph, 2nd section...
"FIRING SUPPORT WEAPONS
A squad without casualties may fire using its Full Firepower, but will be unable to fire Squad Support Weapons. Or, it can fire using its Reduced Firepower and fire one support weapon. Or, it can forfeit its
small arms fire and fire both support weapons."
The firepower of the LMG in the Rifle/LMG stands is already factored into the value we list for the stand. You are given the option to fire the LMG alone using the reduced firepower number and fire the Squad rifles separately also at the reduced firepower. The intent is to allow targeting of 2 separate targets but at the reduced firepower for each. If the stand is a Rifle/2xLMG then the stand could fire both LMGs separately, each at the reduced firepower, but could NOT fire the rifles of the squad. For game mechanics (simplicity and playability), it is assumed the squad is serving both LMGs in some fashion.
Hope this helps!