I believe Tom was the most even headed man in the family, only Mike was better with his emotions than Tom. He was simply thinking business when he wanted to make the deal, he would have made the family money and on top of that avoided a war, so in that thinking Sonny was the onr who needed to handle his emotions. Tom also knew his plac when it came to the family, he did what the Corleone in charge at the time told him to do, which most the time was to give his opinion on a matter. The only people in the family Tom was weak around was Sonny Mike and Vito, everyone was his junior in the family.
I thought Tom kept his cool during his kidnap. Tom doesn't have a choice to be strong because he will never be the Don.
Have you seen Part 2? Tom was the don the whole time Mike was running around the country and in Cuba.
