Yes, cannabis is a short day flowering plant, that means outdoors it flowers during the fall/winter when the days are getting shorter. And yes, so the 18/6 is for simulating the long days of summer and has the plant grow in an aggressive “vegetative” growth cycle where the plant devotes most of its energy into building the size and structure of the plant and then later with the 12/12 light cycle it does indeed “trick the plant”, so to speak, into thinking winter is coming and therefore it starts putting all its energy into producing flowers.
I did a quick google search on the plantmax and they are 3000K, this is a light with a lot of red in it that can be beneficial during the flowering cycle.
This is what I would do with your setup, I’d get a 400watt HID MH/HPS they sell on e-bay for like $100 and use it in this tent and use the T5 lights in the corners for side lighting. I’d take the 6500K and the MH lights for veg and then the HPS and 3000k lights for flower. You can use just the T5s alone without any top lighting and still get decent results in that tent, but with the 400 watt HID combo you could have some seriously dense sticky nuggs coming out of that setup.
If I used only the T5s I’d use the 6500Ks for veg and then add the 3000ks for flower, so you’d run all 8 T5s during flower. Obvioulsy in a 3 foot tent you are not going to be putting 4 foot lights horizontally above the plants. Like you can and would with the 400 watt HID light and only one color set of the T5s in the corners.