Sub-monolayer thin film morphologies obtained by deposition of size-selected CoxPt1-x clusters on graphite have been analyzed for different values of x. In all cases, the preformed clusters can easily diffuse on the surface and gather to form islands of clusters. By changing the cluster stoichiometry, very different morphologies can be obtained, going from large ramified islands to "bunches" of non-contacting incident clusters. We put into evidence that the introduction of platinum atoms in the incident particles drastically changes the interaction between clusters and offers the opportunity to control the coalescence process between them. In this way, by modifying the cluster reactivity, a local self-organization of size-selected magnetic nanoparticles can be achieved.