Stacking - well you're in space so it's not like there's a limit on room
What you do is shove ships into fleets (which can grow larger when you get the right techs) and then the fleets can stack infinitely, but a fleet can only launch one or two attacks per turn (though can defend any number of times).
Diplomacy is the game's weakest aspect IMO, which is sad because some of the race ideas are smurfing golden. If there was ever a game that would benefit from a GalCiv2-style diplomacy this is it, but sadly everything is very impersonal and superficial.
(this game is infinitely better than Civ 5 but then unless you plan on getting SotS2 then upon buying any 4X you will probably have a better game than Civ 5)