Micro-phase separation in mixed Langmuir monolayers of n-pentadecanoic acid and perfluoroundecanoic acid is observed due to chemical dissimilarity between the hydrogenated and fluorinated alkyl chains. We studied the influence of various experimental parameters - including mixing ratio, n-alkyl chain length and temperature - on the morphology of the mixed monolayer using Brewster angle microscopy and atomic force microscopy. In addition, we characterized the role played by a third line-active molecular component, i.e. "linactant". A linactant molecule plays an analogous role to that played by a surfactant in 3D. Linactants tested include partially-fluorinated molecules and two-tailed molecules. We observe that the presence of linactant at very low concentrations systematically reduces characteristic domain sizes, consistent with a decrease in line tension. A quantitative characterization of the fundamental line activity of the linactant is performed by measuring line tension isotherms in the mixed monolayer. The measurement involves the observation of relaxation dynamics of the domains, initially deformed from their equilibrium shape by surface shear.