Establishing a working definition of the complexity of linguistic structure and the accompanying operations responsible for generating these structures is one of the hallmark challenges of formal approaches to language. Advancing any sort of heuristic to measure complexity comes with the caveat of ensuring that it aligns with other established desiderata of a given framework. The challenge is even more daunting when modeling the grammars of multilingual speakers, including the development of heritage languages. Adopting Miestamo's (2006, 2008) systemic definition of complexity, we provide an overview of how the connection between atomic linguistic elements can be neatly captured in an exoskeletal model of grammar. An exoskeletal model calls for a separation of the mechanisms responsible for generating syntactic structure and the insertion of lexical items into said structures. We illustrate the advantages of modeling complexity and its developmental trajectories in heritage languages by taking a closer look at gender assignment. We introduce and develop the core components of an exoskeletal analysis of these findings, sketching how this architecture can provide a novel and conceptually appealing way of capturing complexity in (heritage) language.