I can see both sides of the argument on whether or not Triborg's variations should be rated seperately.
On one hand, Triborg does share some base normals across all four variations which is no different than any other character -- some of which have radically different playstyles and movesets as well.
On the other hand, no other character in the game has so many variation specific strings. In most cases, a variation will have 1-3 variation specific strings -- if any at all. In Triborg's case, he has, what, 5 BASE strings and no shared specials at all? That's including the X-Rays, which no other character has variation specific X-Rays.
My two cents, for what it's worth, which is nothing, is if your tier list is based on characters as a whole and not comparing all variations against one another, then there is no reason to make an exception for Triborg. Triborg's variations being based on characters from previous games should have absolutely no impact on a tier list. None. Otherwise, where's Mournful's special tier placement?