Username rules

The first letter of a MediaWiki username is always capitalized (watch out for conflicts). They can be up to 255 characters long (Not sure what phpBB's limit is, but you probably don't have any user with names that long anyway). Underscores and spaces are considered the same thing (again, watch out for conflicts). And per Manual:$wgLegalTitleChars, "The list of illegal characters is as follows: #<>[]|{}, non-printable characters 0 through 31, and 'delete' character 127)."

I think that's all.