When you drop the feed dogs then the stitch length regultor doesn't make any difference.
The stich length screw/lever ajusts how far foward and back the feed dogs move so when they are dropped doesn't matter how the stitch length is set because the feed dogs will not come into contact with the fabric.
You will have to manually feed the quilt sandwich through when the feed dogs are dropped.
This foot....
http://ismacs.net/singer/quilter.html was designed to do straight line quilting, it is opened toed so that you can see where you are going and the bar/guide on the back slides so that you can adjust it to get a certain distance from the last line and assuming that your 1st line of stitching was straight and even all the rest would be also.
You can use the regular foot that you use with your machine with no problem, don't need to get a fancy something or if your storage attachments are inasseable then don't worry about it and go with what you have on hand.
Make a practice sandwich and see what works for you.
Try it with the feed dogs dropped and also with them up. I have some machines that don't have the ability to drop and find that they work well to quilt with.
Because the sandwich is thicker than the piecing part of quilting then you will probably have to decrease the amount of pressure on the foot.... that should be in the manual... it's the screw on the top of the machine on the left side, furthest to the back... if you need a picture of exactly where it is let me know and I'll try and dig up one for you.
I have heard that some of the new new machines will tell you to fiddle with your tension, I would leave it be and see how it stitches before you start messing with it though.
here is a link to manuals if you don't have one,
http://ismacs.net/singer/manuals/index.html
the one for the 15-91 might will give you some insight and also may want to take a look at the "even feed / walking foot" one which can also be used for quilting if by chance you have one. (still don't necessarly need it to quit with so try what you have (the regular foot) before you feel or get pulled into the 'you just have to have XYZ object to sew or quilt'
