The lore of Wicca in terms of recorded knowledge isn't that existant until recently. I actually have a lesson in my classroom you should view as I just touched this subject on it today
. Anyways it was largely passed verbally to keep a sense of secret, if it was recorded it was usually locked away in a book of shadows of the coven or person.
The crosses differ greatly. The Celtic cross was introduced into popularity by St. Patrick. as the "Driver of the snakes from Ireland" Snakes was another word for druid then. He stole it from the Druids which used it assumingly to worship the sun. As that knowledge of its proper use was destroyed during the Dark Ages in which St.Patrick existed and perpetuated himself.
The Crusader cross was composed of A different form of celtic cross which was absorbed by the Greeks, in turn became a symbol of the crusades. The four other crosses represented the cardinal direction and the 4 gospels of the new testament. Some argue it might have been the five wounds of cross, hands, feet and the javelin to the stomach/heart region which is what killed him.
The cross in general wasn't only celtic, it was Egyptian too. Only known to them as the Anhk which represents eternal life and rebirth. Sounds rather similar doesn't it?
Most crosses have quite a similar meaning, generally stolen or absorbed from Celtic, Roman, Greek, and Egyptian beliefs.