With the goal of achieving an understanding of the properties of bimetallic alloy clusters having atoms of two isoelectronic elements, we have studied the structural, electronic and magnetic properties of MnmTcn, MnmRen and TimZrn clusters with m + n = 13 (n = 0, 1, 4, 6, 9, 12, 13), using first-principles density functional calculations. MnmTcn and MnmRen represent clusters of isoelectronic series with a half-filled d shell, while TimZrn represents an isoelectronic cluster series of early transition metals. Mn-rich alloy clusters are found to prefer compact structures and isoelectronic Tc-rich or Re-rich alloy clusters are found to adopt open structures. In contrast, TimZrn clusters are all found to stabilize in compact structures, irrespective of being Ti-rich or Zr-rich. This change in behavior between two isoelectronic series is found to be driven by differences in hybridization effects, due to differences in the evolution of the relative energy positions of the d level with respect to the s and p levels upon moving from 3d to 4d or 5d elements. This effect further competes with the magnetization effect to decide the morphology of the alloy clusters. Focusing on the magnetic properties of the studied clusters, we find that the single Tc atom substituted alloy cluster exhibits markedly improved magnetic properties compared to that of pure Mn clusters.