Even though Rome had been technically Christian for a century or so prior to its collapse (western empire at least). I assume it would have still been some time before Chrisitianity was universal around the empire?
Were any peoples or groups known to have held on to the belief in the roman deities post-collapse?
The Roman Empire was of course not entirely Christian by the end of the fifth century, but it was pretty close. Pagans were never wiped out, but pretty much all our sources for this period and later were written by Christians, so we can't be entirely sure if they are accurate in their accounts of surviving pagans or not. I've written fairly detailed answers on paganism in the later Roman Empire and the early Byzantine empire here and here, outlining in broad terms the evidence for pagans in a Christian empire. If you have any questions, I'll do my best to answer them :)