Don't forget, while your trailer has dual 3500 pound axles, you have to factor in the trailer weight itself, which is probably around 1500 pounds. So in theory, you can only put 5500 pounds on top, making the Mahindra too heavy. Both the Kioti and the Kubota will get you near the max weight also.
I have the same problem & I am planning to upgrade to a larger trailer as soon as I can afford it. My LS puts me right near the max weight also, just with the tractor, loader, and whatever I have on the loader. That doesn't include any extra attachments either.
As for the tractors, they are all good choices. The Kioti will give you the best value of the three.