This paper assesses how the modelling of a large hydrogen electrolyser load affects the stability of an isolated, converter-dominated power system, using combined Root Mean Square (RMS) and Electromagnetic Transient (EMT) simulation. The objective is stability assessment at the pre-feasibility stage, where detailed equipment models are unavailable, with RMS results validated against higher-fidelity EMT simulations. The electrolyser characteristics a stability model must retain are identified and justified. Two representations are compared in a 100 % inverter-based modified IEEE 9-bus system: a simplified aggregated load model and a grid-following model of an active front-end interface. Simulations of generation outages and network contingencies address frequency stability, voltage response, synchronisation in weak grids and converter-driven oscillations, with control gains varied across scenarios. The assessed stability depends strongly on load representation, control tuning and electrolyser participation in ancillary services. The aggregated model yields optimistic results and omits the synchronisation loops and the voltage and frequency support of converter-interfaced demand. RMS modelling captures most of the dominant dynamics, which are confirmed by EMT, except for the fast converter interactions. Overall, coordinated inverter-level voltage and frequency support with electrolyser participation in frequency regulation proves the most robust configuration, and a basis for future isolated, inverter-dominated systems.