I have a Profit Sharing Plan with 4 long-time participants. One of these 4 terminated during the year with less than 500 hours service, so he does not share in the contribution per document provisions. Can I count this person's compensation when computing the 15% deduction limit? This is just an old-fashioned profit sharing plan, no 401(k) features. Thanks for all input.

No, compensation for the 15% deductibility calculation includes only those who benefited under the plan. In a profit sharing only plan that means the person must receive an allocation of employer contribution or forfeitures.

How about a related question that there seems to be disagreement on: Does the profit sharing deduction limit get reduced by deferrals of participants eligible for the K portion, but not the profit sharing, e.g. terminate during year? I think not; others think yes.

if ee could have deferred, but didn't, the ee is still treated as includable and benefitng. you include him in your adp test. you include him in your 410b. So,you get to include his comp. (maybe??)
The ERISA Outline book (page 7.200 - 7.202) (5th edition)
says the answer is not clear. conservative approach is to exclude terinees comp, but arguably, since you include him everywhere as I noted above, it could be argued to include the terminee with less than 500 hrs. some good points are addressed
remember the regs weren't written with 401k in mind, benefitting is what the 404 deduction seems to hinge on.
I have always included the comp, but don't really consider that being agressive. for example, what if the ee deferred 1 cent. now all of a sudden you include all his comp. that makes no sense to me.

I agree with Tom. For 401(k) plans, we include all who are in the ADP test when calculating 404 comp. We don't consider it particularly aggressive, either.

To further complicate the question...What if you have two plans. the first is a 401(k) with 3 months of service requirement and the second a Money Purchase with a year of service requirement. What compensation is used to caluculate 404 limit?

you want complicated? suppose your document says top heavy is given in the money purchase plan. you now have ees who worked 3 months. they are participants so they are eligible for top heavy. yet they are not in the money purchase. yet you have to provide a top heavy minimum. poor plan design.
again, it shouldn't matter how many plans you have, and if there is a difference in eligibility. add up all 'benefiting ees' and use that as comp. (less of course any deferrals , including 125 deferrals, then cap comp at the $ limit)

Tom, although it is a little off topic, do you put the top-heavy into the (k) for these individuals rather than the money purchase?

I honestly don't know the best way to handle it. I just remember reading about 'make sure you don't do this', but I don't recall if there was an answer. I think you would have to follow the terms of the document, e.g. if it says top heavy is provided in the mp, then provide it in the mp.
maybe this would require a corrective amendent stating that
'in addition to the mp contribution, top heavy minimums would be provided to those employees who are eligible for the 401k plan but are not participants in the mp plan'