When is it grammatically correct to create a contraction of words followed by "is"? For instance is "who's" a correct short form of "who is"?
The word "is" can always be contracted, provided it is not stressed - though this is considered somewhat informal.
So, in informal contexts,
can be shortened to
...it doesn't depend on what word comes before "is". But if the is is emphatic:
then it would be wrong to contract that to
That is probably fairly obvious - if you are stressing a particular word, it doesn't pay to shorten it. But there are also other situations where the is has the emphasis, for instance
cannot be contracted to
can be reduced to
Who's is the correct contraction of who is, in the same way let's is the contraction of let us.
There are no grammatical rules about not using 's as a contraction of is.