If you are going to keep the barbarians in the game you have to be very carful what unit slots you use. I have writen a paper describing all the techs and how they relate to barbarian appearance.
http://mrtemba.civnation.com:666/bp.doc
There are a couple of ways to fix the problem, but the best is to have something more appropriate then a jet fighter in the crusader slot.
Another is to give the industrialization tech at the start of the game, this will jump the barbarian order of appearance over the crusaders to musketeers and cannon slots.
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"Bite the wax tadpole."
-Coca-Cola as originally translated into Chinese
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