If the current version of Numbers is 6.2.1, then you cannot download an earlier version. When you upgraded to Sierra on your Mini you were able to upgrade to Numbers 5.1. Perhaps were you able to upgrade the MBP, then you may have been able to upgrade Numbers.
Apple no longer supports older software versions once they have upgraded macOS and, also, have upgraded the software. They are trying to push you into buying newer hardware. To use the same software versions on different devices, the devices must all be upgraded to the same OS version required for the current versions of the iWork apps. If you are able to upgrade the MBP to Sierra, then you should be able to use the Numbers version you use on your Mini. You don't have to download it. You can copy it from the Mini to the MBP once it is running Sierra.s