Co-simulation is a useful approach in the modelling of robotic systems composed of multiple parts. In co-simulation, the subsystems only exchange information at communication points. The time delay of information exchange may cause error and instability. Thus, an appropriate way to determine the interface variables between the communication points is essential for efficient and stable performance, especially for real-time applications. Reduced interface models (RIMs) can be used to represent the dynamic behaviour of the subsystems at the interface in co-simulation. Such a model-based co-simulation scheme was limited to systems consisting of rigid bodies in previous studies. In this work, we introduce the formulation of RIMs for flexible multibody systems and based on that propose a general co-simulation scheme for systems consisting of both rigid body components and elements with structural flexibility. A robotic model is employed as an example to demonstrate the co-simulation scheme, where a non-smooth subsystem with contact interactions is present. The advantages of constructing RIM using flexible mechanical system models over rigid body models are also addressed by comparing the effective mass properties and the simulation results.