By means of atomistic molecular dynamics simulations, we investigate the structure and stability of alkyl-modified polyamido-amide (PAMAM) dendrimers at the air/water interface as a function of the number and the relative position of the modified end groups. We find that the PAMAM dendrimer with all terminal groups functionalized is more stable at the interface than the Janus dendrimer where only half the amine groups are modified. This result is explained in terms of softness of the dendrimer molecule which adapts its conformation to maximize the favorable amide water contacts segregating the alkyl chains and increasing the structural order of the "hydrophilic core". We interpret the stability of the molecule at the interface also in terms of particle wettability and energy components. We verify that the atomistic model follows the prediction of the thermodynamic analytical theory adopting an oblate or prolate shape and orientating its longest axis parallel to the normal to the interface. These results indicate that monolayers of fully functionalized molecules could be as stable as (or more stable than) those self-assembled from proper Janus molecules.