Most heterogeneous modeling environments allow only one model of computation at each hierarchical level of a model. The consequences are that (1) the transformations that occur at the boundary of two models of computation depend on the modeling tool and (2) the hierarchical structure of the model is perturbed by layers introduced to allow changes of model of computation. We introduce "heterogeneous interface components" to allow flat heterogeneous modeling. Such components have inputs and outputs that obey different models of computation. We present an execution model that allows the use of these components with any model of computation.