He certainly didn't seduce Elizabeth. And there is no evidence that he seduced Mary King. Also, do we know that Mary King was young? His "penchant for young girls," as you call it, may have been a tendency to work on the affections of young women without much fatherly protection close by.

The rumours in Meryton, that implied that Wickham had seduced all the young ladies with whom he might have been connected is likely to be completely (or at least mostly) untrue. It seems that everybody wanted to be able to say that they had always suspected he was no good when in fact prior to his leaving Meryton, people seemed to like him. Popular opinion is very fickle. :-)