This paper proposes a Decentralized Integral Single-phase Sliding Mode Control (DISSMC) for Load Frequency Control (LFC) in Complex Power Systems with Multi-Source generation (CPSMS), integrating reheat, hydro, gas, and wind turbines. A generalized structure is employed to model Multi-Area Linked Power Systems (MALPS), providing a realistic representation of diverse power plants. The proposed method formulates an integral Sliding Surface (SS) to mitigate the chattering phenomena and ensures finite-time stability through a continuous control law. Additionally, a single-phase technique eliminates the reaching phase, ensuring immediate trajectory control. The MATLAB/Simulink simulations validate the DISSMC's effectiveness in stabilizing frequency fluctuations and managing load variations across three interconnected regions, each hosting different power plant types. The results highlight the proposed controller's robustness and adaptability to dynamic load and renewable energy source fluctuations.