Green hydrogen will play a key role in the decarbonization of the steel sector via the direct reduction path [1]. To meet the demand side, both a highly efficient numbering-up based scaling strategy for water electrolysis is needed as well as flexible operation strategies that follow the fluctuating electricity load. This paper presents a modularization approach for electrolysis systems that addresses both aspects by combining different electrolysis technologies into one heterogeneous electrolysis system. We present a modular design of such a heterogeneous electrolysis system that can be scaled for large-scale applications. The impact of different degrees of technological and production capacity-related heterogeneity is investigated using system co-simulation to find an optimal solution for the goal-conflict, that the direct reduction of iron for green steel production requires a constant stream of hydrogen while the renewable electricity profile is fluctuating. For this use-case the distribution of technology and production capacity in the heterogeneous plant layout is optimized regarding overall system efficiency and the ability to follow flexible electricity profiles. The simulation results are compared with conventional homogenous electrolyser plant layouts. First results underline the benefits of combining different technologies and production capacities of individual systems in a large-scale heterogeneous electrolyser plant. It is shown that the presented modular electrolysis systems can follow the current flow without losing significant efficiency. However, for a downstream system that requires a constant hydrogen mass flow, supplementary hydrogen storage is required.