'vehiclemodelsets.meta' is pretty flexible. You don't need to stick to what is there, you can add in new vehicles to sections & the game will randomly choose which one to spawn etc. They are spawned using scenarios, so it's even possible to create completely new locations & specifics of how something spawns using CodeWalker if required etc.
At some point, if you add tons of new vehicles to 'vehiclemodelsets.meta' the game will go crazy & start swapping vehicles about. You'll see stuff like the game trying to take-off in SUV's on the airport runway & such. You just lower the number of vehicles in there to fix it.
Also seen errors in 'vehiclemodelsets.meta' edits that corrupt 'update.rpf'. So backup the complete 'update.rpf' before editing