I'm going to second those who said that different canons should be allowed, but different versions of the same character from the same timeline shouldn't. There's just too much potential for things to get REALLY messy if you've got multiple versions of the same character from the same canon running around. For instance, if a Kharlan War-era Yuan were to join, he'd likely find out about Martel's death and Mithos going nuts and would probably want to do something about it... but if he did that would throw off the timeline for MY Yuan, who is being played according to canon events. Otherwise you'd have to say that the two of them were alternate universe versions of each other, which is just opening up another can of worms entirely.