NEW VERSION of the question:
Let me try to be very precise about what I'm looking for, incorporating here what I have written at a comment to the earlier version of my question. As in the illustration to be found here, I want to produce a number of arrows
->, <-, <->, =>, <=, <=>
all looking exactly like, resp., LaTeX's \rightarrow, \leftarrow, \leftrightarrow, \Rightarrow, \Leftarrow, \Leftrightarrow, and also produce, in a standard way, the arrows
>-, -<, >-<, >=, =<, >=<
which are not native to LaTeX but should look like the arrows above, except that they invert the directions of the arrow heads.
It is extremely important that these new arrows should scale in a natural way, just like the native LaTeX arrows, when used in subscripts or superscripts.
(Notice that smooth scaling is fundamental, but only part of my problem.)
OLD VERSION of the question:
In the answer to this question I learned how to produce some new arrow symbols that I was in need of. I would now like to make these symbols scalable, so that they could be used in subscripts or superscripts without ad hoc adaptations. Can anyone help?