i have seen a similar way of changing an adjective into a noun, but instead of SA they use MI. Can i use both? Or is there a special rule for using each of them?
さ is used to express objective properties of something, and み is used to describe perceptions - the way something makes you feel. For example, 暖かさ can be measured with a thermometer; 暖かみ is how warm you personally feel. In metaphorical situations, like the "warmth" of someone's personality, you would use み, not さ.
Hahaha! たまらない can be used to mean irresistible, such as たまらない愛嬌 (irresistible charm), but also unbearable, such as たまらない暑さ (unbearable heat). I suspect the second meaning is the intended one, rather than what is indicated by the translation...