Hello, since your question is not directly related to SSA course which this forum is specifically about I'm going to recategorize this posting to the Pega Platform General questions.
Correct, we recommend keeping integration assets in a separate ruleset. This is because those may need to be versioned at a different time than the implementation layer.
Yes, it is reasonable to put data assets in a separate ruleset if there is a versioning benefit to do so for a specific application or for the purpose of sharing that same ruleset across multiple applications. It's more about the benefit of the assets in a separate ruleset than the type of asset.
Remember some data rules also go into org rulesets. Most of the time implementation work and data compliment each other and that is the reason of putting them together. As in any other architecture INT is isolated as they are reusable across applications in any organization.
In a typical large organization, the org rulesets contain all data classes across the org. You can further have another org data ruleset apart from ORG in particular if it is needed. ( I assume in rarity) .