As one of the authors behind the clinical toolbox, I would be delighted if people used my work and cited my paper. However, users should be aware that the template developed by Rorden et al. (2012) was specifically designed for use with SPM. While generally ignored, it should be noted that SPM tends to generate average sized brains, while tools like FSL and ANTs come with templates that are MNI template sized. The MNI template is larger than average. As noted when the template was released, this was a side-effect of the algorithm used to develop this template (there is no evidence that people in Montreal have larger than average brains). This difference is best seen in Figure 1 of Horn et al. (2017). This difference explains why MRIcroGL’s File/OpenStandard menu lists mni152 as well as spm152. The former template is for images normalized to MNI sized brains, while the latter is for average sized brains.
As you note, human brain volume shrinks on average about 5% for each decade after 40. Therefore, it might be worthwhile using an older template.
I really do not know how the regularization tuning influences different tools. I do know the images developed by Rorden et al were intentionally designed to work hand in glove with SPM. For more general algorithms, I suspect there are better options, e.g. those from John E. Richards