I have been quilting for customers for 15 years so maybe I can answer your questions.
What is the size of your top in inches? My machine is 15 feet long; however, the leaders that I pin the sandwich to are 150" long. I could pin a 150" backing onto the leaders, but I would not want the quilt top to come to the edge of the leaders, so I would say a top around 144" long
When someone says that all the threads should be removed from the back, they usually mean that those little stray threads either attached or loose might show just under the quilt top after it is quilted (doesn't look very well). If the fabrics of the pieced top are fairly dark or busy those little threads probably won't show. But a light or white background will probably let them show, therefore the reason for removing them.
A little pressing can coach most seams to lie down nicely.
If there is no border, basting around the outer edges will insure that the seams do not come unsewn in the process of handling and loading the top onto the frame.
I have found that I can deal with an inch or two off square on a large king quilt and have it quilt up nicely. More than that the top might end up with a small pucker, or a little pleat to work the extra fabric in.
Hopefully this will help a little bit.