Rear derailleurs come in different speeds and cage lengths. Let's break down what we know about each:

7sp rear derailleurs must use freewheels, which are really low end, and wobble on the back of a bike, shifting can never be precise. Hence these are considered junk in the industry.

8sp rear derailleurs should use cassettes ( gears on the back of a  bike ). Therefore they are way better than the 7sp stuff.

9 & 10 sp derailleurs are awesome. 11sp are not commonly found on folders, so it doesn't apply here.

There are three different cage lengths for a derailleur: short, mid, and long. Long cage derailleurs don't make much sense on most folders. With a wide range cassette the long cage derailleur can make ground strikes ( a big no-no ). Therefore only riders that tour should consider a long cage on a folder.

A short cage derailleur will minimize ground strikes, however it will limit the range of the cassette 11-36 is not possible on a short cage system.

Therefore a mid cage derailleur is ideal for most folding bikes. It will minimize ground strikes, and will allow 11-36 rear cassettes. Additionally, only buy a bike with a replicable rear derailleur hanger, this is the weakest part of a  frame, making it replicable is wise. We have swapped most of our lineup to mid cage rear derailleurs with replacable hangers to take advantage of these benefits.