Personally, I usually wait 6 wks to the day, but that's just preference. My levels are usually all zero at 4 wks, and would probably be fine to add a fish at that time, but I wait. If it's been a month and your levels are zero, you should prob be fine to add your chromis. Just remember: add your fish slowly... let the tank get use to the chromis and then add your fish one at a time - slowly. Make sure to wait with any fish that may be more aggressive and add them last. On my first tank, I added a yellow tang as my 4th fish. I wasn't able to add anything after that. It didn't bother any fish that were already there, but wouldn't allow any more. I had to trade him.
Well.. good luck! And post some pics when you've added!