We have one PS plan that has a two year of service and age 21 for eligibility requirements. Once eligible, of course, 100% vested immediately. Does the 1000 hour rule apply as it typically does for the usual one year/age 21 plans? One employee has her two years in but only worked 973 hours each year and I want to confirm she's still not eligible. Thank you.

Your plan document should specifically address this issue. I know of no reason that a year of service can not be defined as any year in which an employee has worked 1,000 hours or more just because there is a 2 year waiting period. But again, your plan document will give you the definition of a year of service.

yes, but be careful how document is worded.
Most documents define first year date of hire to date of hire and then after that you switch to plan year.
so technically an ee hired DEC 1 could work 13 months and have completed 2 years of service!