Myacin effects the gram-positive bacteria. Whereas denitrifying bateria in a reef tank are gram-negative and are not normally effected. With that said I would dose in very small amounts and give it a day to see if it worked before increasing the dosage. Your skimmer will go nuts when you turn it on. You may want to follow up treatment with a significant water change.

I don't think any amount of flow will completely keep the stuff from growing. There always seems to be some spot where it will grow regardless. I have had it grow right on the nozzel of my return pump.