Yes, you have bacteria built up. There is no doubt about that. The question is do you have enough bacteria built up to handle five fish added all at once to a brand new system? This is what is concerning.
That is a lot of fish to add all at once to a newly cycled tank. For a 650 tank (a tank that size) adding two fish every 3-4 weeks shouldn't cause any issues, but adding 5 fish all at once has the potential to cause issues.
Keep in mind that ammonia is known to be very toxic to marine fish (LC50 value below 1 ppm).
Ammonia readily passes across the gill membranes and rapidly enters the blood of fish. Its effects include damage to the gills, resulting in poor gas exchange, ion regulation and blood pH regulation. Other effects include hampering oxygen delivery to tissues, disrupting metabolism and toxicity to the nervous system that causes hyperactivity, convulsions and death.
My suggestion is to take some sort of corrective action if the total ammonia rises above 0.1 ppm.
To be on the safe side, have lots of already made up salt water on hand b/c you may just end up getting a mini-cycle in the new tank. If this happens you will want to replace ~50% of the water. If you get a ammonia reading of 4ppm, then replacing 50% of the water will lower the ammonia level by 50% so you would lower 4ppm to 2ppm. And at 2ppm, this level can still harm your fish.
Another option is to have on hand products that bind the ammonia to make it non-toxic. There are several on the market (e.g., Amquel or Seachem Prime). Personally I wouldn't use them in the DT, only in QT. But, sometimes it may be worth using this in the DT is it means saving your fish.