Hi Justindo and Richard, I think you are both right, the flag stickers may save the making of non-pirate ships. I agree that pirates and not soldiers are the favourites of childs (and not-so childs), because of old movies. I think that just with flags and some isolated sets with soldiers (of which one has to buy many copies to get an army) the problem can be avoided. Navy ships and pirate ships would differ just by flags and crew, but not much problem.
Playmobil made of many XVIII century sets with soldiers, and I am happy with that. What I would like is a big non-pirate ship (I would tend to think the most powerful ships were owned by any nation's navy, and not by lawles pirates: I would prefer bigger ships for British soldiers and smaller for pirates -unlike the smaller old British clipper).
Merchant ship crew do not need to be much different from that of pirate ships (perhaps one can select for merchant ships "pirate" clickies without wood legs or hooks in the arms).
Sorry, my Anglo-saxon fellows by being somewhat Hispano-centric (it's difficult to avoid cultural preferences), but I think a good enemy for pirates would be that made classic by old movies, the Spanish Gold Galleon. Morrion-using old enemies (not the XVIII tricorne-using adversaries), as made famous in movies (I love morrions). These big ships were both heavily armed, permitting to fight back pirates, with soldiers in the crew, and at the same time were ships carrying a cargo of American Gold to Europe, so being a temptation enough to pirates to attack even when they had to fight (a normal merchant ship would be less armed and easier to defeat).
Well, this being said, one can said that the Playmobil pirates, because of their tricornes, are more likely of the period when they mostly confronted British and French navies, the Spanish being far from their XVI century naval hegemony.