I could not get it apply to some indicators despite following the excellent instructions, which led me to discover there are two kinds of indicators. Maybe others knew this already, but it took me a while to figure it out.
This tip applies to the application indicators (aka appindicators), which are mostly third-party apps plus Network Manager and keyboard/ibus.
The sound indicator and messaging menu (and power, clock and user on Unity, I think, but not on MATE) are system indicators. There is a way to reorder these too, as described here (note that it works for me in 16.04 and 16.10 despite it saying 14.04):
What you can't do, as far as I can tell, is intermix the two types of indicators.