I definitely think these weapons deserve to be in the game, as I'd find their absence more of an immersion breaker than seeing a large group all armed with FAL's (Or the less plausible but still possible SCAR 17) and the PKP definitely has a place, right next to all of the other Russian/Soviet weaponry. Although, the chance of finding one should be pretty low, if not for realism, balancing. Just my opinion though, I could very well be wrong about that.
Mihail Volkov was previously enlisted in the Russian armed forces, serving under Spetsnaz for 6 years before he was discharged for an undisclosed reason, although it is known to fall under disorderly conduct. After being discharged, he started working as a gunsmith, making his previously learned skills useful. He moved to Chernarus for his job, hopping between city to city. Eventually, he was brought back onto military installations as a civilian consultant, but was kept on a short leash. After security relaxed, he began smuggling spare parts and ammunition off base, selling them or keeping pieces to build guns for his personal collection. Eventually, he was found out, but before they could arrest him, he had packed up and moved shop. He jumped from city to city, until the apocalypse was a bigger concern than evading authorities.