So, Captain-Kingsman16 asked me what would be good figures to use if I wanted to make customs that would match the Jakks Pacific Sonic figures, so I put some thought into it. We would need a figure that is about 2 and a half inches tall at the shoulder (the head can always be swapped so it's less important) and doesn't have heavily defined musculature. I think the action vinyls made by the Loyal Subjects might work since they have similar articulation and proportions, here's Froppy from my collection next to Shadow to show an example.

https://www.deviantart.com/wakeangel2001/art/Comparison-Of-Figures-856349443