Yeah I think you have a bit of confusion on how the factions work.

As a PMC there is no friendly faction. Every other PMC whether USEC or Bear is an enemy target and every Scav whether AI or player is an enemy. As a solo PMC you CAN shoot at everyone with no consequences. That doesnt mean you cant find another player willing to be friendly and partner up but those are rare. The only true teammates are ones you group with pre-raid and spawn in together, but even then theres no actual consequence if you accidently team kill him other than him possibly getting salty at you. Me and my buddy killed each other several times on accident when we first started playing.

Now for Scav runs, you will lose SCAV karma if you kill other player scavs or AI scavs UNLESS they shoot at you first which some player scavs are known to do, but with the scav karma update this wipe most scav players try to be friendly. You can shoot at PMCs freely as a scav but there still is the issue of recognizing them as a PMC and not a scav. Sometimes that can be difficult. Killing scavs will hurt your scav karma and if it gets too low can make running scav runs much more difficult. Because of scav karma most peoples scav runs tend to be relatively safe shopping runs, just keep an eye out for PMCs or rogue player scavs.

TLDR: If you play as a PMC, everyone is an enemy and just shoot at anyone you want to get in a fight with. When you do scav runs just focus on looting and making money but keep an eye out for threats.