I've never had any problem with mixed ancestry, and honestly don't understand why anyone would. There's no such things as 'pure' any clan, because the clans aren't some eternal thing, they were just started one day. Do we count descendants of Blue Whisker (Moth Flight's ThunderClan daughter) to be not 'pure' TC? Even though Blue Whisker was one of the earliest ThunderClan cats? What about Thunderstar himself? He doesn't have TC blood because there was no ThunderClan when he was born. It's all just splitting hairs. I doubt there are any cats alive who can only count original members of their Clans as ancestors.