Is this eligibiltiy for 401(k) ok?

Plan has this elgibility:

Age 20 1/2

1 YOS (1,000 hrs)

Entry date:  First day of plan year following.  That's it.

Ont he surface it seems it violated the 18 months rule, but I am half-remembering (probably incorrectly) that the age 20 1/2 somehow buys me an extra 6 months, but I can figure it out.

No, this isn't permissible. I believe you may be thinking about a plan that uses age 20-1/2 with a 6 month eligibility and a single entry date. That would be ok. But in the situation you have, a person might not enter until after the statutory entry date requirement.

Same rationale for a PS only plan, right...

Freddie the Unlucky is hired 1/3/2017. age 30 or 40, depends on the age of the lady he is talking to at the bar.

he works 1000 hours in 2017. the regulations say 6 months after meeting 1 year of service or age 21.but his next entry date is 1/1/2019

I'm guessing they could use the 2-yr eligibility with that entry date, tho'

I don't think so. You could have 1-1/2 year eligibility, as long as you 100% vest immediately.