Going to a different breed can be done, but requires even more caution. Ideally you'd want to use something that the breed was created with, or as close as you can get (if you're trying to do something like a new variety). With the Delawares for instance, it makes zero sense to outcross to Columbian anything, Delawares are NOT supposed to have color in male saddle. Now if there were decent Light Sussex out there that might make a little more sense due to them being on wheaten base pattern and therefore not having that color in the saddle. However that introduces the problem of white skin, which is dominant, so it would take two generation to get rid of, there's some type variance as well that would take a couple years to fix.
This right here is one of the reasons why I enjoy talking chicken with @BGMatt
People tend to offer unsolicited advice in the "Why don't you just ...!" argumentative tone. And after talking to Matt I have some handy answers.
Judging from the number of places people can buy Delawares with white legs, I'm assuming Light Sussex has been well-tested in various Delaware breeding programs. I do really like the overall shape and heft of a nice Sussex, but besides the skin color, they're also a bit, well, dumpier than what I'm going for with the Delawares.