the 5 you mentioned.
1. we know nothing about the so6p, although he was definitely insane. but the fact that he had to split the juubi into 9 means that he did NOT manage to tame the 10-tails, and therefore wasn't a perfect jinchuuriki.
2. those 5 can break chibaku tensei together
3. numbers overwhelm, even if one on one the so6p is on a whole different level. he cant fend of s-rank and a-rank techniques all the time.
i expect the so6p to take them to the very edge though. but it's a bit ridiculous to suggest that the so6p could beat 5 top 10 ninja of all time at once, doesn't make sense to me.
on the other hand, if they do manage to kill the so6p the juubi will return eventually and they will have another massive problem.