Background/Objectives: Avocado (Persea americana Mill.) seed is an agro-industrial by-product with significant pharmacological potential. This study aimed to conduct a preformulation assessment of aqueous seed extracts from Criollo (M1) and Hass (M2) cultivars. Methods: The extracts were characterized for total phenolic content (TPC) and ABTS^•+ scavenging capacity, and their compatibility with pharmaceutical excipients was evaluated in binary mixtures by Fourier transform infrared (FTIR) spectroscopy with Pearson correlation and hierarchical cluster analysis (HCA), differential scanning calorimetry (DSC), and isothermal stress testing (50 °C, 15 days) with HPLC quantification of epicatechin (IST-HPLC). Results: M2 showed significantly higher TPC (192.80 ± 4.90 µg GAE/mg) than M1 (27.08 ± 0.51 µg GAE/mg; p < 0.0001). ABTS^•+ inhibition was 59.92 ± 1.76% for M1 (0.05 mg/mL) and 50.79 ± 1.19% for M2 (0.01 mg/mL). The DSC transition enthalpies of both extracts did not differ significantly (p = 0.1271). Maltodextrin and gum arabic were compatible with both extracts, suggesting that they could serve as common excipients for both cultivars, whereas the chemical stability of five M2 mixtures could not be determined by IST-HPLC. Sodium carboxymethylcellulose (CMC) was incompatible with M1, as shown by a 29.44 percentage-point decrease in epicatechin recovery (p < 0.0001), separate HCA clustering (Euclidean distance > 0.009), and the lowest FTIR spectral correlation (r = 0.990). Conclusions: These findings provide a scientific basis for the rational development of solid dosage forms from avocado seed extracts.