Personally I think you should be fine keeping them all, just don't veg them too much longer, LST, let the lower branches grow out and flip, you could tie and flip.
It wont hurt to go bigger on the pot size. 5gallon is standard these days, I use 15ltr and don't have any issue. I tend to use 6 plants in a 4x4. I have used 16 in 1m2 before looked the size of yours in those 6.5ltr pots, flipped as they are now. Didn't have a problem. But I don't like having 3 lots of 16 plants. Its personal preference Angel Eyes.
With 9 I would transplant up and tie down straight away, veg a week or 2 and flip. But I'm sure dave would do different, neither of us are wrong. If you had 1000w I could say veg them longer and flip untrained but a 600w would give a better coverage over shorter plant trained with more colas. Thus allowing your light a little closer too.
Honestly though, don't stress over it. one way or another you will get some good weed. Potting up would be the best option as to not restrict the root growth and therefore not restrict the yield.
I'm sure you will get other opinions too. Theres a million ways to grow awesome buds!
. It fills with 6 plants in 5 gallon pots. I generally top th main and LST the whole plant bending it horizontal to the ground. Attaching to the side of the pot. After the flower stretch undo the tie down and let'em fly
