"Who(m)ever I hire should follow the rules." I think "whomever" is correct, right ?
Afraid not! 'Whoever' (nominative) is correct: the concessive pronoun is the subject of the verb 'should', with insertion of the contact clause 'I hire'.
WARNING: NOT A TEACHER
(1) An excellent teacher has given you and me the answer.
(2) I should imagine that 99.99% of native-speaking teachers of English would agree
with him.
(3) Learners come to usingenglish.com because they want to speak modern English
that is used in the real world. The teachers here teach you the English that is actually
spoken in the year 2011.
(4) In fact, Mr. Michael Swan in his highly reliable Practical English Usage (1995 edition)
wrote these words: "whomever is not used in modern English." (page 623)
(5) NEVERTHELESS, I agree with you. According to the rules of traditional English,
you are 100% correct. The "correct" sentence should be: Whomever I hire + should
follow + the rules.
(a) I suspect that most non-native-speaking teachers of English (who have been
schooled in traditional English grammar) would also agree with you.
(b) If you want details explaining why I feel that you are correct, please answer this
post, and I will be happy to give a lengthy explanation.
It is correct, and Michael Swan may be slightly overstating it to say that whomever is not used nowadays, but it is so rare that whoever sounds much better to me. Even in sentences like Give it to whoever you see that tick a number of boxes for it, whomever would sound artificial to me.
On reflection, I am inclined to agree. I inadvertently treated the sentence as elliptical for
Whoever it may be that I hire should follow the rules.
hence my designation of "I hire" as a contact clause.
However, as the sentence stands - and as unnatural it may well sound - objective-case 'whomever' is indeed justified here.