You can reference objects into a different scene file. Look up referencing. But if it's not too complex a scene, just model everything in one scene, it's simpler that way. If you want to reuse a model from an old scene, you can just export it as an obj, for example. UVing happens after modelling and before texturing... not too sure the distinction you're asking about? You can model everything, UV everything, and then texture everything, but then you've got all your UVing to do at once. Some people UV as they model (so model something, UV it, model something else, UV it). Just depends on what doesn't drive you crazy.