If you want your color slides scanned and saved in a specific order then they should be numbered on the slide mounts in sequencial order. Then group them in stacks of 50 or 100.

You can fit approximately 1000 slides onto your DVD Slideshow but I would rarely recommend using this many. Peoples attention spans are not that lengthy. Sometimes less is more. You would be far better off editting out just the best images to keep peoples interest longer.
If slides are of poor quality it will just detract from the overall presentation. A good number would be between 200 and 500 slides.
This will give you a running time of about 20 to 55 minutes.

The scanning software we use can auto adjust for those issues but in severe cases it may not be enough. In some cases a custom scan may be necessary.

Yes, We can scan 126 and 127 slides as well. There is a little higher cost with those sizes. If your job is not a slideshow and we are just scanning and saving to a disk then we will scan all the 126 size slides together as a separate group.