Skimming collects only the highest version of every rule in the rule set and copies them to a new major or minor version of that rule set on the same system, with patch version 01.
Before deleting lower versions of rule set , please check if you have all required rules present in the latest higher version of the rule set. If not, you will face rule not found exception during Rule resolution.
Skimming is only for rule resolved rules, meaning rules which have ruleset versions. But Data- instances associate only with the ruleset, but not any ruleset versions. You can check any of Data-Admin-AppID or Data-Admin-System-Settings instances. Since Data- instances don't associate with ruleset versions , you can delete lower ruleset versions.
as far as I know the data instances that are in a RuleSet (e.g. classes) won't be deleted if you delete the lower RuleSet version. Additionally data instances associated with a RuleSet (e.g. users) won't be deleted as well.
I would assume that it is possible to delete the lower RuleSet version if everything above is skimmed correctly. Hence I'm not sure I would do the following:
Export the application and all related data instances only with the higher (skimmed) RuleSet Versions
Import these in a separate environment, e.g. a virtual machine
Run the application validation wizard and look for missing references.
The riskier way would be to delete the lower RuleSet Versions and check withing the system itself. You should make sure to keep the products generated by the deletion wizard, because you can use them afterwards to re-import the deleted rules.