Sub-monolayer epitaxial self-assembled silicides of Er and Gd on Si(111) exhibit unique structural and morphological features that play a crucial role in their magnetic behavior. The effects of disorder occurring between the characteristic quasi-1D surface reconstructions of such systems were analyzed by low energy (LEED) and reflection high energy (RHEED) electron diffraction. The observed Rare-Earth Metal (REM)-Si antiphase domains caused appearance of diffuse intensity planes in the surface reciprocal space, visualized as straight (arched) streaks in LEED (RHEED) diffraction patterns. Random registry shifts between adjacent adsorbate atomic chains caused extinction of the half-order satellites in diffraction patterns of a surface reconstruction identified by scanning tunneling microscopy (STM) as Si(111)-(3 x 2)-Gd.