Teams of heterogeneous mobile robots are a key aspect of future unmanned systems for operations in complex and dynamic urban environments, such as that envisions by DARPA's Tactical Mobile Robotics program. Interactions among such team members enable a variety of mission roles beyond those achievable with single robots or homogeneous teams. Key technologies include docking for power and data transfer, marsupial transport and deployment, collaborative team user interface, cooperative obstacle negotiation, distributed sensing, and peer inspection. This paper describes recent results in the integration and evaluation of component technologies within a collaborative system design. Integration considerations include requirement definition, flexible design management, interface control, and incremental technology integration. Collaborative system requirements are derived from mission objectives and robotic roles, and impact system and individual robot design at several levels. Design management is a challenge in a dynamic environment, with rapid evolution of mission objectives and available technologies. The object-oriented system model approach employed includes both software and hardware object representations to enable on- the-fly system and robot reconfiguration. Controlled interfaces among robots include mechanical, behavioral, communications, and electrical parameters. Technologies are under development by several organizations within the TMR program community. The incremental integration and validation of these within the collaborative system architecture reduces development risk through frequent experimental evaluations. The TMR system configuration includes Packbot-Perceivers, Packbot- Effectors, and Throwbots. Surrogates for these robots are used to validate and refine designs for multi-robot interaction components. Collaborative capability results from recent experimental evaluations are presented.