I had a similar issue in my grow tent in my last grow. you can see it in the 'indoor' section under "Testing the Pro-Grow 260 LED light".

I downgraded the size of my tent and I had about 8 plants too. What I found was, having less plants in the tent actually gives more yield !

I killed off all the plants which had the least amount of growth. If you take the plants out you might see that 4 or 5 of them will be very underdeveloped.

The reason for my plants growing tall was because they were all fighting to get the light. Imagine a forest, and in the forest there are large trees, the smaller trees on the bottom of the forest floor must grow upwards quickly to get at the canopy for more light. These smaller plants are in a race to get to the canopy to take all the light. This is what is happening in your grow room. the plants are in a constant battle to get over each other.

When you take the plants out you look for the 3 or 4 strongest plants and keep them, find the lesser plants which hopefully do not have much bud on them and put them outside or something. Anywhere but in the tent. You will notice that with 3 or 4 plants left in the tent the bottom of the tent has plenty of light reaching it. Your plants, if positioned equally under the light, will now stop fighting and growing over each other and focus on growing some good bud.

I made this mistake myself and I missed out on a lot of bud as the plants were competing. You need to make the grow room less stuffy or risk losing the potential of much more bud.
You can see what happen to me in my grow. I saved the day by pulling the majority of them. I only grow 4 plants in my 90cm x 90cm x 150cm tent now. They will always stay at a reasonable height as long as I don't overcrowd.

And you should keep the autos on 18/6. The more light the plant gets the better your yield. But giving them a 6 hour break is good for them. Personally I would go 20/4 but that's just me.

Good luck and i hope this advice helps

S.L.