If you are rotating your tires correctly they should all wear out at the same time. If just 2 are warn you have a mechanical issue that needs to get fixed. You should get this fixed before you spend money on new tires that will also wear out.
I do not like to mix tires. Mixed tires will give uneven traction and could cause a control issue.
As long as the tire sizes are close in diameter, the sensors will not trigger anything, its when the sizes are way off in height they will go nuts, you can use any size you want as long as they fit in the wheel well.
Like if you ran 205's and 235's, they might go nuts, but I don't think you could get much bigger than a 225 on the car without lifting it.