When should I use App Studio or Dev Studio to build out assets on the subclass, such as what was shown for during the Polymorphism topic?
Once you have created the required subclasses, and they are available in App Studio as Data Types, you can work with them in App Studio in the same way as you would the top-level superclass: add Data Model, add Views, configure Data Views.
Marking rules in your superclass as "Relevant Records" that you expect to be class-specialized will encourage them to show up in App Studio for extension.
Start off by checking in on Dev Studio that changes you make in App Studio are landing in your expected Classes & Rulesets. As your confidence grows, you will find yourself spending more time in App Studio.