Mixing of religions was not something that was ever encouraged by Christians all the way back to St. Paul...If anything, the evidence of the little mixing there was (similar feast dates etc that you mention) were just a small bone the pagans got out of it. IMO, the progressive 'idea of mixing' is not something to be encouraged now or in the future. And if there is a pure form of litergy/practice that already exists in places like Africa and India, we should protect it from external attempts to gain control and influence in our churches/religion.
When it comes to actual mixing (e.g. Christianity + Voodoo), you are correct.