Combing floating breakwaters with wave energy converters (WECs) and integrating them into very large floating structure (VLFS) can provide a viable option to explore economically offshore wave energy resources and simultaneously to protect marine structures. In this paper, the time-domain numerical model is developed based on the modal expansion theory with nonlinear consideration to optimize the design and layout of an integrated system of modular WEC-type floating breakwaters and a pontoon-type VLFS, with emphasis on the effects of the WEC geometric size and shape, the WEC-VLFS gap distance and the wave nonlinearity. A hybrid finite element (FE)-boundary element (BE) method is presented to simulate the structures as Mindlin plate elements and the water waves as fully nonlinear potential flow boundaries, respectively. Breakwaters as WECs with deeper draft and larger length are found to more fully interact in phase with long-period waves, and receive more wave energy extraction and larger hydroelastic response reduction. The addition of breakwaters has a favorable effect on the wave energy extraction, but a destructive effect on the hydroelastic reduction. Importantly, wave resonance induced by the multi-modal scattering waves in the WEC-VLFS gap leads to multiple peaks of the power capture efficiency. Compared to the symmetric-shape WECs, the asymmetric-shape WECs strengthen the gap resonant effect, which improves both the wave energy extraction and hydroelastic reduction for a broader frequency bandwidth. The findings of this study indicate the synergistic benefits of wave energy exploitation and transmitted wave attenuation at the fore-end of VLFSs.