What are the levels of derailleurs (Shimano probably) I know Altus and Acera but don't know the higher end ones. I am putting this on my Sondors Fat ebike. I will be using a 7 speed freewheel, and a single sprocket in the front. 750 watt motor, 52 volt battery so I need strong chain, freewheel and derailleur. Thanks in advance.
The higher end groupsets (Alivio and above) would not be the right Speed as you plan a 7 speed freewheel and they are all 9 / 10/ 11 speed.
Shimano's webpage has in the Trekking section: Acera - 9 speed, Alivio - 9 speed, Deore - 10 speed, Deore XT - 10 speed
In the Mountain bike range there is (lowest first): Tourney TZ, Tourney, Tourney TX, Altus, Acera Altus, Acera, Alivio, Deore, Zee, SLX, Deore XT, Saint, XTR
I think the only derailleurs Shimano would say that work with 7 speed would be the Tourney rear derailleurs and the Acera RD-M360 8 speed derailleur, unless I've missed some.
But I can't see a reason that a 9 speed rear derailleur wouldn't work as the actuation is the same. The cage may possibly be fractionally narrower as its designed for the narrower 9 speed chain but I have heard of people running this with no issues. You would have to use a 7/8 speed chain though and not a 9 speed one.