We compute fluctuation corrections to the Flory-Huggins theory for a thermal or nearly athermal polymer mixtures. These excess free energy contributions arise from architectural and flexibility differences between components, are quasi-universal, and are generally positive, favoring demixing. Specific cases considered are binary blends of linear homopolymers, homopolymer/block copolymer mixtures, and blends of branched and linear homopolymers. We identify invariant parameters and mixing rules for such systems that may facilitate the design of miscible polyolefin blends.