With the new Responsive Design in 7.6 will I be able to set a Max Width without any problems?
For each style which you want to have a max-width set to, add this to the EXTRAS section at the bottom:
#content{margin:0 auto;max-width:1050px;}
Adjust the "1050px" to your preferred value.
The "auto" part of the margin property will center the forum on the page.
Will existing images included in posts using [img] tags automatically be resized?
Images within the post will also automatically be resized to never display larger than the width of the post. Even hot-linked images should be resized for you forum.