X460G2 is preferred over X670V, because it has a better control plane (better CPU). Master role is CPU intensive, so it's logical to set the best CPU as Master. In terms of dataplane, no matter which one is the Master.
Mixing x460G2 with x670V is not a bad idea, especially considering SummitStack-V160. That's a lot of bandwidth available for stacking links.
As for the various HW limits, please note that EXOS is smart enough to optimize resource usage. It's NOT as simple as "down to the lowest capable".