The amt of nutes will depend upon how healthy they are for sure. Yours look excellent right now. I have run mine up to 2.1 - 2.2 EC this time around.
I don't think you should just jump to those levels though. If you plan on growing them bigger than gradually up the EC as they get bigger, and as long as they continue to look healthy.
Keep good notes so you know how old they are, size, and amt of nutes you give them.
6-9" I would be around where you are now on EC. They will grow fast now for sure. Its always easier to give them more, not so easy to fix if you gas them with too much nutes.