Custom menu – different submenu for first menu item

Well, the answer depends on what you mean by “custom dropdown only for the first menu item which is different from the other item dropdowns” – different how? If it’s just stylistic, that extra class should be enough. You just have to make sure the css for it is specific enough to override the defaults. So if your css selectors on the other dropdowns are

.nav li.has-children a{ ... }
.nav li.has-children>a { ... }
.nav li.has-children:hover .subnav { ... }

then to override them for just that first item, if I’m understanding your nav markup right, you’d need something like this (after your existing css):

.nav li.settingsitem a { ... }
.nav li.settingsitem > a { ... }
.nav li.settingsitem:hover .subnav { ... }