Why would you give them a pistol instead of another rifle? If the point is that five rifles is too strong, you can just make the rifles weaker, because you're making an entirely new squad.
Do you want riflemen to have lugers available to loot and take home with them? Is that it?
Good one .
The luger on spawn would serve for lowering down their DPS since they should be available early and offering a way for an weapon upgrade different to the LMG.
A mid to close weapon (could be 1-2 weapons) could also work for allowing them to fight CQC units a bit better since their DPS would not be enough to deter units from storming them. Hence the grenade also.
The unit should be used more as helping normal grenadier with something more durable than completely replace them.