Multi-robot formations have numerous applications, such as cooperative object transportation in smart warehouses. Here, robots must deliver objects in formation while avoiding intra- and inter-formation collisions. This requires solutions to multi robot task assignment, formation generation, rigid formation maintenance, and route planning. In this paper, we present a cooperative multi-formation object transportation system which explicitly handles inter-formation collisions. For formation generation, we propose a distributed motion planning approach which combines artificial potential field methods and leader-follower based control. For formation planning, we present a heuristic search-based algorithm which uses convex segmentation techniques, and extend the minimum snap method to synthesise smooth trajectories while maintaining the formation. We also propose a variant of the dynamic window approach to avoid collisions between formations. We demonstrate the efficacy of our approach in simulation.