Your suggestion does make sense in that it would allow Guards to keep some of their "elite"-ness (by not being gimped by forced PTRS), and also their uniqueness (by being the only squad capable of picking up 4 weapons and becoming generalists)
What I'm concerned about is that if you start with 2 DPs, skip PTRS (which aren't that fantastic), and manage to pick up other LMGs from the ground, you might get a terminator squad.
You could offset that by making Guards 3-slot units and making PTRS worth worth 0.5 slots. However, even with 3 DPs, Guards become really, really scary.
That fear does rely on units dying and dropping lmgs though. If anything the dp28 is the one lmg that is balanced for having 2.
Any unit with 3 lmgs is indeed powerful. Just like getting g two lmg42 on grens or lmgs on obers: extra lmgs translate very strongly into effective non mobile AI. But again dp28 aren't exactly lmg42 or lmg34s. Those are already terminator weapons in the hands of allied squads.
If the ptrs upgrade remained filling 2 slots, that upgrade would prevent an lmg pickup. That terminator guard squad would immediately find itself unable to deal with any kind of armor. Guards that find lmgs would certainly be very heavy on the AI. But they'd be incapable of defending against axis armor.
Brits dropping guards bren would be strong as well. I get this.
However I do think the fear of guards that manage to find dropped lmgs is less of a concern than the current makeup of Soviet infantry functionality.