Vesting from Effective Date of the Plan

I'm setting up a DB and 401(k) for a prospect.

All employed on the effective date enter the plan on the effective date; anyone hired after, 12 months of service.

Plans will state Vesting from the effective date of the plan - DB has to be -3 year 100%, but PS will use 2/20.

I do recall a thread mentioning this would not be possible?

I can't speak to what you may or may not recall, but I don't see any problems with anything you described.

Agreed. You can start vesting from plans' effective date and for BRF purposes, the statutory vesting schedules are deemed equivalent.

Has the plan sponsor ever maintained a plan before?

No, and I do know the rules concerning this issue.