If a plan has a 90 day wait for eligiblityand defines a part-time employee as an employee who works < 19 hrs/week, can they exclude part-time employees? This is under the assumption they pass 410b w/ the exclusion.

my understanding is that you can't use the term part-time and exclude them.you could however write the document to say something like

completion of 300 hours in a 3-month period or 1000 hours in a 12 month period which should accomplish the same thing, unless you have 'part timers who work a lot of hours for a few months and few hours the remaining months.

You could define eligibility as 90 days for those not classified as part-time employees (defined however you want) and 1,000 hours during any eligibility computation period for employees classified as part-time employees.

Yes. There are a lot of plans out there that exclude part timers from early entry dates but have a "savings clause" that allows them in if they work 1,000 in the year. And the plans got determination letters.