The seven requirements are important when paralleling transformers.This is when the transformers primarys are connected together and their secondaries feed the same load. When you are banking transformers
like three transformers feeding a three phase load or two transformers in an open delta configuration the impeedance is not important. The only exception is primary delta and secondary delta. Then you want to have your impeedances close. In the case of the open delta with the 50kva and the 10 kva you woutd be just fine. If you bank a transformer with two others the one with the lower impedance will take more of the load. Being that the impedance of the 50kva is most likely lower than the 10 kva you are in good shape.
