The disk stars are the youngest since they are the most metal rich. The key to understand how our Galaxy formed is the location, ages and chemical composition of the various stellar populations. The oldest stars are in the halo and bulge. The most metal rich stars are in the disk and bulge. From this we deduce that the halo formed first, followed by the bulge then disk.
Inelastic collisions occur when two objects collide and share momentum as a single body.
But gas clouds are large and can 'stick' together. Even the above facts, the formation and evolution of our Galaxy must have taken place through a series of continuous stages. First, the Galaxy began as a large single gas cloud a few hundred of thousand light-years across. Passage near other proto-galaxies caused this large cloud to spin. This rotation was far from organized as currents and smaller clouds formed within the proto-Galaxy.