Don't go over 1000 mg APAP dosing at once (4000 mg in a day but taking it at once is heavier so don't go over 1000), so DXM + APAP pills if containing that amount would be fine to add to the DXM-only pills to reach 600.
15 mg is really very little. If you only want to add that 50 mg DXM (60 actually, with the 4 pills) to the 550 to reach 600+, I guess it's a little too much taking 4 of those pills but should be fine. Stick to 3 if you're unsure but it's only a minor crossing of the instant dose limit (1300 instead of 1000).
It probably won't feel terribly light on your body and it isn't great for the liver, but if you do something like this incidentally (assuming you have zero liver problems!) then you should be okay. But don't make a habit of it. Buy DXM only products or extract the DXM.