You can not go in reverse with a 66, the thread will break. You can sew a few stitches, stop and lift up the presser foot to move the fabric back to the beginning of the seam then start stitching again. I'm not sure if I would try that with a walking foot. If I remember correctly you can't sew in reverse with a walking foot. If your machine has the back clamp feet, I don't think you will find a walking foot.
The 99 is the 66's little sister, same machine but the 99 is the portable version. If you measure the distance from the needle to the pillar there is less room on the 99.
You have to keep in mind that both are domestic models not industrials. They will do lots of heavy work but there is a limit.
Hope this makes sense and welcome to the QB!