For application to thermal barrier coatings (TBCs), ytterbia (Yb₂O₃), ceria (CeO₂) and niobia (Nb₂O₅) are investigated for their effect as co-dopants on the stability of 7 wt% yttria (Y₂O₂)-stabilized zirconia (ZrO₂), known as 7YSZ. Zirconia TBCs must be stabilized to prevent transformations between cubic (c), tetragonal (t) and monoclinic (m) phases during operation, since the latter causes cracking and failure. However, with an increasing use of co-dopants in industry, very little data exist on the cooling rate sensitivities of the stabilizing mechanisms involved. Thus, that is the focus of the current study, together with microstructural and mechanical effects. Co-dopants are added to separate samples of 7YSZ and three cooling schemes are applied. It is found that the stabilizing mechanism of lattice distortions is the most sensitive to cooling rate, and that of oxygen vacancies is relatively insensitive while providing roughly four times more stability against m phase. Additionally, it is shown that m, t and c phases can be identified on fracture surfaces by intergranular, textured transgranular, or smooth transgranular crack propagation, respectively. Furthermore, co-dopants are found to influence Young's modulus according to their cationic properties, their effect on oxygen vacancies and their effect on the amounts of phases retained: Young's modulus is decreased by cationic radii larger than that of Zr⁴⁺, increased with raised concentrations of oxygen vacancies, and decreased with an increased presence of m phase.