Materials with the alluaudite structure have been attracting increasing interest due to recent reports of promising performance of Na2+2xFe2-x(SO4)3 as a sodium-ion battery cathode material. In previous work, how selenate doping could be utilized to expand the range of alluaudite phases that can be synthesized is highlighted and an ionic size relationship is proposed to determine whether an alluaudite (Na2+2xM2-x(SO4)3-y(SeO4)y) or 2-1-2 (Na2M(SO4)2-y(SeO4)y) (M = Co, Mn, Ni) phase would form from dehydration of the initial Na2M(SO4)2-y (SeO4)y.2H2O precursor. In this paper, this work is extended to investigate mixed transition metal (Fe-Ni, Mn-Co and Mn-Ni) systems to confirm the applicability of the proposed ion size-structure relationship. In agreement with these prior studies, the smaller transition metal ions (Co and Ni) naturally adopt the Na2M(SO4)2 (2-1-2) structure, while the larger transition metal ions (Fe and Mn) adopt the alluaudite-type structure. By exploiting the ion size design relationship, an Fe-containing 2-1-2 composition has been successfully synthesized with the aid of Ni-doping. Furthermore, for the Co, Ni systems, it has been shown that by co-doping with Mn, the synthesis of Co, Ni containing alluaudite phases can be achieved with lower selenate doping levels than required previously. Thus, this work further illustrates the ability to exploit cation sizes to direct the structure obtained for these sulfate/selenate systems, which allows the design of novel compositions for potential future cathode material applications. The characterization is also reported of the related Na2Cu(SO4)2-y(SeO4)y phases, which adopt a different structure, most likely attributed to the Jahn-Teller nature of the Cu2+.