This paper addresses cooperative manipulation and transportation of any payload shape, by assembling a group of simple mobile robots (denoted m-bots) into a modular poly-robot (p-bot). The focus is made in this paper on the chosen methodology to obtain sub-optimal positioning of the robots around the payload to lift it and to transport it while maintaining a geometric multi-robot formation. This appropriate positioning is obtained by combining the constraint to ensure Force Closure Grasping (FCG) for stable and safe lifting of the payload and the maximization of the Static Stability Margin (SSM) during the transport. A predefined control law is then used to track a virtual structure in which each elementary robot has to keep the desired position relative to the payload. Simulation results for an object of any shape, described by a parametric curve, are presented. Additional 3D simulation results with a multi-body dynamic software validate our proposal.