Maintaining plasma uniformity is an essential requirement for successful x-ray laser designs. In this work we focus on a Z-pinch-driven neonlike krypton x-ray laser design for which we (1) investigate the role of initial mass loading in affecting plasma uniformity and gain and (2) show that there are advantages in terms of plasma uniformity to diluting a krypton plasma with a low-Z material such as helium. These results are obtained by using a one-dimensional radiation hydrodynamic model. The results of this study show that low-mass 100% krypton plasmas are optimal for achieving significant gain while maintaining plasma integrity. Diluting a krypton plasma with helium has the advantage of improving plasma uniformity, but it has the disadvantages of enhanced collisionality and line broadening, which are associated with the additional free electrons.