If I add 1.5 inches to my current wheels, is that how much they will stick out?
No it wouldn't. The widest part of your current set up is the sidewall where it bulges as the 205's fit on the 5.5" rim. Assuming you are keeping the same tyres and going to 7" rims then the sidewall will become less bulged so therefore thinner overall so you may find they will stick out less.
However the other thing that will determine where they sit will be the Offset of the rim, the mounting face, of the brand you purchase. All things being equal with the wheels you have now then the offset would only 3/4" to outside width, the other 3/4" would go in under the guard accounting for the total of 1.5" extra in the rim. You would need to learn the offset of the rim you have and compare it with the 7" rim you would like to have.
Sorry you are still no closer to knowing that you can just buy some rims and they will fit without issues.
Terry