Is there any way to add multiple objects to a particle simulation (emitter) without having to join the objects before adding them to the sim?
You can put all objects you want to emit in one collection. In the Render settings of the Particle System you can choose Collection which will use all the objects in this collection.
By default, it's picking single objects to be emitted as particle. But you can check Whole Collection which means, each emitted particle will then consist off all objects in this collection in their relative positions like they are placed to each other.
For emitting single objects of the collection you can also check Pick Random, although it already looks random to me without this enabled.
Another setting you might want to use is Use Count, there you can specify if some of those picked should appear more often than others from the collection.