Microsurfacing is a pavement preservation treatment that applies a thin, rapid-setting mixture of cationic asphalt emulsion, fine aggregates, and a polymer-modified binder to restore surface condition and extend service life. This study evaluates whether crushed volcanic and fluvial-derived aggregates exhibit measurable differences in performance when incorporated into microsurfacing mixtures, and examines whether meeting mechanical screening criteria alone is sufficient to ensure satisfactory mixture behavior. Four aggregate sources from the Quito metropolitan area were characterized using sand equivalent, Los Angeles abrasion, and sulfate soundness tests in accordance with ISSA A143. Only two sources satisfied all mechanical thresholds and were subsequently incorporated into mixtures with a polymer-modified cationic quick-set emulsion (CQS-1H-P). The resulting mixes were assessed for mixing time, early mechanical development using the TB-139 Wet Cohesion test, and binder–aggregate compatibility through the Schulze–Breuer–Ruck (SBR) method. Findings indicate that aggregates meeting cleanliness, hardness, and durability requirements may still perform poorly during mixing or early setting if chemical affinity with the binder is insufficient. One mechanically compliant source failed to achieve minimum cohesion thresholds and exhibited unstable mixing behavior, whereas the other demonstrated stable consistency and high SBR compatibility. The results underscore that microsurfacing aggregate selection must integrate mechanical screening with chemical compatibility evaluation to ensure reliable and durable field performance.