There are countless legendary enemies between raiders and super mutants. I have also noticed that there are legendary and higher tier versions of soldiers in the B.O.S. Is it possible to find higher tier (such as the name is different[ example. knight commander instead of knight]) or even legendary minutemen?
Probably there are no higher ranked or legendary minuteman.
I too have seen legendary units of BoS and the Institute, while allied or at war, but I am assuming this is because you CAN be at war with those. But, since it is not an option to be at war with the minuteman (with the exception of Pinned), so there is no scenario in which you will be fighting them on a regular basis.
Also, on the fallout 4 wikia there are only three base ID's listed, two for male, one for female minuteman, and none for a higher ranked or even legendary version.
This could be explained by the fact that you are rebuiling the minuteman, so there hasn't been much time to put back together the old structure.
To finish off: I have never seen any, neither at the castle, as random spawns or as support called in by the flare gun.