How can I have a fixed order of the menu-extras? Like the OSX menu items for wifi, battery... I would like to be able to have a permanent order of the third-party menu bar items.

I do not want to mix OSX with third-party menubar items, I simply want them to always appear in the same order.

Setting an order for the menu extras to launch in the startup items does not do the trick.
You can hold command while drag and drop it to change the order. I don't know it will remember the order or not though.