The sequence of authors is not all a major issue if there is good understanding between the authors. The problem arises when there is forceful intrusion without any contribution. In my opinion the concept of first co-authors is good.

it is good, shows the ability of each individual to collaborate as a team. but several first authors is not that much acceptable !

in m own opinion, it all depend, but I don't see any difference in it provided that all author contributed equally.