I use nutes with Ffof once they are growing well. For me the indicator for when to start is when the first set of true leaves start to show signs of hunger (yellowing, getting thin and silky). Hungry plants show signs from the bottom up.
If you give nutes to a plant that is in drier soil they suck it up faster and get a nute rush (cool to watch in a more mature plant). This is a good technique but in a plant with only two or four leaves it can burn that layer. If the new growth looks ok that's probably what it was.
Your plants look great and you are getting good advice. I would not worry too much.