Unless the rules changed and I missed it (sadly possible given my lack of activity of late) there is no ban on clans. There is a ban on clans that are exclusionary and make things not fun for everyone else. Specifically, "Don't use your clan to ruin other people's fun".
For instance, by your definition, and substituting IRC for Skype, a whole group of people I know were a clan. Originally played together in various ways, all of us ended up with characters in Arcaea, chatted in the #Arcaea channel, and quite a number sent people to Pian en Luries when Koli became active/powerful there. However, others were included in the power discussions, and while most of the Arcaea bunch started supporting Koli and he them, various divisions arose (best example is probably De-Legro's and Solari's characters at the end), and I don't think anyone would argue that there was undue amounts of working together, given that everything was dissolving into civil war even before Koli's assassination.
What do you mean, I was helping Koli the entire time, he just didn't really know what was best for him. Solari and Juan ALWAYS knew what was best for Koli
