Thanks to @Relaxed 's helpful comments I now know at least the power situation: right now you might find yourself on a train where only first class has power but this is changing and at some indefinite point in the future every seat will have power. Details:
The SBB timetable change on 14 December 2014 says:
From 14 December, passengers will be able to travel southwards and back again in increased comfort, as ETR 610 trains will be operating on the Gotthard route. In future, a total of 15 of these trains will run on the Gotthard and the Simplon routes. These trains have [...] power sockets by every seat
But it also says
They will gradually replace SBB's ETR 470 trains
This Tripadvisor post claims that normally all Zurich-Milan trains run on the Gotthard route that the SBB post mentioned.
About the ETR 470
in first class, power sockets were available
There's a video of an ETR 470 Zurich-Milan train posted on December 31, 2014. Another video also shows an ETR 470 on this route on January 13, 2015. So they aren't out yet.
The new ones, however, has been running for some time. The Zurich-Munich direction trains are EC 13,15,17,19. A photo series of the EC 13 on shows the occassional ETR 610 back in 2013 but later photos show ETR 470 again. My conclusion is that there is no consistency and you can't plan -- if you need power, buy first class or wait until SBB announces the ETR 470 gone. Edit: this happened at the end of 2015. And even the ETR 610 is not for long, by 2019, EC 250 trains will run on this route.