A regiment that is not issued lasguns is quite simply an accident. And as for the different marks and patterns ... not even the US manage to equip all of their infantry with the exact same gun , yet I wouldn't say they don't practice standardisation. Ironically enough, the Imperial Guard is probably in a better situation than the US military here, in that at least all the lasguns and laspistols seem to be able to take the same chargepack, whereas the US don't even manage to shoot the same bullet, let alone use the same magazines.
In guns of Tanith, the troopers are issued chare packs that aren't compatible with their tanith lasgun.
Which is among one of many reasons why I don't think Gaunt's Ghosts are really 40k novels, but generic military sci fi with a 40k sheen over it. That bit made no sense. The major element of the lasgun is that it should be an easy thing for the Imperial Guard to keep supplied. The different patterns of lasgun don't matter so much, but they should all accept the same charge pack. Also, as mentioned, the charge packs should be rechargeable, so why didn't they just recharge the ones they had? Because Dan Abnett wanted a story where they are short of ammunition, and he ignored the canon elements of the lasgun to allow that to happen.
One thought I had to partially reconcile it was that the charge packs as delivered had a "sleeve" on them to fit larger style lasguns more comfortably, which could have easily been removed to make them fit the slimmer style lasguns of the Tanith. However, due to the lack of technical understanding by the characters in the story they were not aware of this and thought the packs would not fit their weapons and they left them behind.
Still fails to explain why they didn't recharge their existing packs...