Is it possible to have a class composed of other classes? Or if not, if I have 20 classes as possible Backtrack in chaining context, is there a way to refer to them in 1 rule, rather than 20?
No, what you want is not possible right now.
You can only combine classes through fea code, which will then be combined into a single class.
You can only combine classes through fea code, which will then be combined into a single class.
Ad hoc or permanently?
It is from fea code to a single combined class.
See the fea specs:
http://adobe-type-tools.github.io/afdko/OpenTypeFeatureFileSpecification.html#2.g
Now that’s very useful to know 
Dank u wel