The design of solid electrolyte composites for Li-ion batteries integrating polymers and organic ionic plastic crystals (OIPCs) requires an understanding of the synergy between their components to achieve the desired ionic conductivity. Here, we study composites between the OIPC hexamethylguanidinium bis(fluorosulfonyl)imide HMGFSI and polymers functionalised with the comonomer lithium 1-(3-(methacryloyloxy)propylsulfonyl)-1-(trifluoromethylsulfonyl)imide (LiMTFSI). These polymers have different macromolecular structures (i.e., homopolymers, copolymers, linear polymers, or polymer nanoparticles) and concentrations of lithium (Li). Characterisation of the composites by differential scanning calorimetry, X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and electrochemical impedance spectroscopy showed that the composite containing large polymer nanoparticles with a low Li concentration had the highest ionic conductivity and structural disorder at low temperatures as well as a higher fraction of ions (i.e., Li⁺ and FSI⁻) that become highly dynamic. The role of the polymer nature and Li content in promoting interactions that led to different ion dynamics in the composites was discussed. Understanding the complex interplay between the composite components and the effect on properties such as thermal stability, structure and ion conduction, and dynamics assists in optimizing the overall performance of solid electrolyte composites.