I'm thinkin', unless, of course, this isn't entirely feasible, either way, those Swiss Pike at the Spanish Harbor would work really well with the Britsh Harbor Highlanders, or, they should, at least, be offered across the board where applicable! :-)
Insofar as, the potential to access allies' decks...or even turn control of army units over to allies! Strictly regarding Harbor units - not civ specific ones. ;-)