Ok, my tit bits, again,
My point was that at the time Kalyavan would have received such a boon, the Yadavas were Mathura based and rather weak. Nobody would be doing yagnas or tapasya to get boons to defeat them! Yeah, later on, due to the successes of Krishna & Balarama and the introduction of new warriors like Pradhyumna, Satyaki, Charudeshna, et al, they became as powerful as the Kurus. But that was not the case at the time Kalyavan would have made his request.And if he was a Yavana, there is no way he could have gotten boons from Shiva or Brahma, since vedic meditation and scriptural info would have been alien to him.