You are in full control, eg can use whatever textures you like. It's up to your game toolkit, but typically texture switches have a cost, as they cause the graphics pipeline to be flushed. It may be worth it to decide what images you need for a screen, scene, level, or similar, then pack a texture atlas at runtime with just those images. Then again, maybe you have few enough texture switches you don't need that complexity.
As for how it works, instead of using AtlasAttachmentLoader, you can provide your own which does not load textures. Later, before you render an attachment, you'll need to give it a texture and UVs. See:
Loading Skeleton Data - Spine Runtimes Guide: AttachmentLoader
