I have added the IV car pack, and edited the popgroups.ymt so they will spawn, but not all cars spawn anymore, only a select few of the cars spawn, from the mod and vanilla. Now there is usually just duplicate traffic of the same car. I also have it starting at the end of my cfg, so all the cars start before it.
Here is the resource
popgroups.rar (14.8 KB)
also, the mod from gta5mods if you want to test it yourself