Aside from impersonation-types of usernames, I'm fairly sure that anything goes--including curse words. I think I've come across quite a few of those kinds of usernames in the past, so the forum isn't exactly censoring selection based on language.
This is an interesting kind of thread and not something I'd ever really considered before, not even that usernames with "satoshi" in them were forbidden. I definitely didn't know that, and it's kind of a nice gesture to reserve that specific string of characters for bitcoin's founder.
There are similar usernames on the forum, like for example darklord & DARKlord, people may assume the two account is controlled by one person or one is the alt of the other. A neutral tag by the older account solves the problem.
I'd say a negative tag is more appropriate if the usernames are similar enough such that there's a high probability that the newer account is trying to impersonate the older one. That's happened a number of times since I've been a member here.