Does anyone see a problem with aggregating a SEP with a DB plan for 410(b)/401(a)(4) testing ? Can you do that ? I realize I'll have the 404(a)(7) deduction limits since I have a common participant, but assuming that isn't too restrictive can I do this ? Thank for any input.

I suggest you do a search. There are several discussions related to this. I believe that the general consensus is NO, a SEP may not be aggregated for 401(a)(4) testing because it does not meet the definition of a "plan" under 401(a)(4).

I thought one of the requirement for a SEP (at least if using the 5305-SEP) was that the employer could not maintain another QP?

That's true on the plan doc issue but this client appears to be using an investment prototype SEP doc and not the IRS model SEP doc.