You can't rest downthrow fair against sheik (can you? that would be so ****)
The fox uthrow uair thing doesn't work unless the fox spaces incorrectly, so there may be percents where it is more likely to happen, but it is very possible it will never happen. Honestly, imo, you should not really try this unless you notice the fox is spacing wrong and like upairing inside you rather than just hitting with his feet.
The reason is that at low percents, at least for me, if i try to do it and they space correctly i smashdi and normal di down so they get a second upair far too often. And at high percents i think its better to smashdi, unless you really need a comeback, but even then, i think you are better off smashdi'ing and living and then gimping or something, its not reliable enough.