HR Help desk new HR Request form - Subcategory selection and Data Validation

• Is there a way to make only sub-categories selectable on the Request Creation page? We would like top-level categories to act as grouping headers only and prevent users from selecting them when creating a request.
• Does the Request Creation page support data validation rules for category-related fields? We have tried configuring a few validation rules, but they do not appear to be working as expected. Are there any limitations, prerequisites, or recommended approaches for validating data during request creation?