Based on DFT results suggesting that Ge should be an interesting dopant, we carried out a detailed study of a prototype Ge-doped Pt cluster catalyst: Pt4Ge/alumina. This cluster was found to have high stability in repeated ethylene TPD experiments, with good retention of strong ethylene binding sites, however, the stabilization mechanism was unusual. Ge addition only partially suppressed carbon deposition, however, the amount of carbon per cluster self-limited at 2 C atoms/cluster, and DFT showed that the resulting Pt4GeC2 clusters were highly stable, yet retained a substantial number of strong chemisorption sites. The obvious question is whether this mechanism might also stabilize the activity of PtnGem clusters of different sizes and compositions. This paper reports experiments and theory on larger PtnGem/alumina cluster sizes, addressing questions such as: Does Ge addition suppress carbon deposition, and if so, how? Is coking self-limiting for the larger clusters, and if so, how does coking vary with cluster size? Does Ge confer stability against both carbon deposition and sintering, and if so, how? Do the effects of Ge (and carbon) addition largely result from simple site blocking, from changes in the Pt electronic properties, or both? These issues were probed by a combination of ethylene and CO TPD, time-and temperature-dependent He + ion scattering, and detailed DFT modeling of Ptn/alumina, PtnGem/alumina, and PtnGemC2/alumina samples. Several minor improvements to the experimental protocols were made in the course of this work, aimed at reducing complications from adventitious adsorbates and improving the calibration of the numbers of molecules desorbing in the TPD experiments. Thus, in addition to the new experiments, we repeated previously published C2D4 TPD experiments for Pt4/alumina, Pt7/alumina, and Pt4Ge/alumina. For these clusters, the desorption temperature dependences were essentially identical in the old and new results, however, the improved calibration reduced the absolute numbers of D2 molecules desorbing by ~30% in the new experiments. Although that is within our estimated 50% absolute uncertainty, the new results are reported below so that direct comparisons can be made for different cluster sizes and compositions.