Yes, it is possible to apply specific approval rules to subsets of requirements.  The key is to apply a Ketryx Query Language (KQL) filter in the advanced settings JSON.  The JSON follows the syntax below:

"itemFilter": "Insert KQL language here"

So for example, if I wanted a specific approval rule to apply only to Requirements with parent Requirements, I would specify:

"itemFilter": "RQ and \"has parent\"::RQ"

The subsequent approval rule would then apply only to this subset of requirements.

Note: the distinction between KQL used in JSON, and KQL used to filter items from the All Items screen, is how quote marks are handled ("").  In the JSON, nested quote marks must be preceded by a backslash (\") for both opening and closing the quoted section.  Otherwise, the JSON script will read the openning of the first nested quote as the closing of the initial quote, which will create invalid syntax.