Stacked Match with 3-month CODA

New 401(k) established 10/1/17 with a 1/1/17 effective date.  Plan includes Safe Harbor 3% provisions as well as a discretionary ACP safe harbor contribution and a discretionary PSP contribution.  Of course, two owners were able to defer the maximum $24,000 before year-end.  The rest of the 6 staffers were able to accumulate about $2,000 each in deferrals before year end.  Is it too aggressive to allocate a stacked ACP safe harbor match limited to 4% of pay using full-year compensation?

If it is legal, (meaning you have crossed every t and dotted every i), it is legal.

Any particular reason for thinking it wouldn't be legal (or allowed)?

I don't know the answer, but isn't there an argument that for the purpose of determining "safe harbor compensation" (you can follow the trail from 1.401(m)-3(b)(3)(ii) to 1.401(m)-3(b)(3)(i) to 1.401(k)-3(b)(2) to 1.401(k)-6) the plan year (see 1.401(k)-6) means just the 3-month period the plan was in effect? 1.401(m)-3(f), which generally requires a 12-moth plan year but allows an initial year to be as short as 3 months, strongly implies that in such a case the "plan year" is only 3 months long.