Despite the huge progress recently made in understanding the phenomena of metal-promoted growth of one-dimensional (1D) semiconductors, the controlled formation of small diameter semiconductor nanowires is still challenging. Liquid growth promoters, such as the low melting Au/Ge eutectic, allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, and operating pressure. However, the incorporation of metal atoms during the growth process, size variations of the nanowires due to agglomeration of the nucleating metal seeds, and surface diffusion of Au via the vaporliquidsolid route have been reported. Here, we detail the influence of solid growth seeds, such as NiGe2 formed from Ni nanoparticles, on the lateral dimensions of Ge nanowires grown using a supercritical fluid growth process. Beneficial control over the mean nanowire diameter, in the sub-20 nm regime, with a predominantly <110> growth direction and low structural defect concentration was obtained using Ni seeds. In addition, the effect of prealloying of NiFe films for the growth of Ge nanowires was investigated, which leads to a bimodal nanowire distribution. Electrical characterization performed on single nanowire devices showed p-type behavior for Ge nanowires grown from Ni and Ni/Fe seeds. Determination of resistivities, majority carrier concentrations, and mobilities suggest significant doping of the Ge nanowires by Ni when grown via a supercritical fluid-solid-solid (SFSS) mechanism.