Co-simulation techniques enable the coupling of physically diverse subsystems in an efficient and modular way. Complex engineering applications can be simulated in co-simulation setups, in which each subsystem is solved and integrated using numerical methods tailored to its physical behaviour. Co-simulation implies that the communication between subsystems takes place at discrete-time instants and is limited to a given set of coupling variables, while the internals of each subsystem are generally not accessible to the rest of the simulation environment. In non-iterative co-simulation schemes, this may lead to the instability of the integration. Increasingly demanding requirements in the simulation of machinery have led to the coupling, in real-time co-simulation setups, of multibody models of mechanical systems to computational representations of non-mechanical subsystems, such as hydraulics and electronics. Often, these feature faster dynamics than their mechanical counterparts, which leads to the use of multirate integration in non-iterative co-simulation environments. The stability of the integration in these cases can be enhanced using interface models, i.e., reduced representations of the multibody system, to provide meaningful input values to faster subsystems between communication points. This work describes such interface models that can be used to represent nonsmooth mechanical systems subjected to unilateral contact and friction.