Genealogists and historians that have checked into immigration patterns into the South before the war have found that it was still mostly English that came over, although the Irish and Scots elements certainly did too.
But if you look at how many flag submissions were based on the English cross and not the Scottish cross (like 6 to 1) then the English element was indeed the strongest. Southern society was set up like English society complete with a class system. "Ivanhoe" and other works of English literature were huge before the war.
Whatever the root symbol of the saltire was, in heraldry it was called a saltire and that's what Miles said. Since he invented the flag I will take him at his word.