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The key to having different benefit groups is that the groups are not as a result of some sort of legally protected characteristic. For example, you couldn't offer different plans to men and women, or people over and under 40, but you could offer different plans to exempt and non-exempt employees.
Caveat - if the basis of defining the groups (like exempt & non-exempt employees) has an inadvertent adverse effect on a protected class, I would caution you of using those guidlines. For example, if all your non-exempt employees are Asian, but all your exempt employees are white, you have unknowingly discriminated, and they could potentially have a claim for adverse impact/disparate treatment.
"You are you. Now isn't that pleasant?" -Dr. Seuss