How do followers of the Hanafi school pray in congregation with followers of the Shafi'i school, given their difference regarding the obligatory amount of sitting in the final part of the prayer?

It is permissible for someone who follows the Shafi'i madhhab to pray behind an Imam who follows the Hanafi madhhab, and similarly for the rest of the four madhhabs. This is because the disagreements between them are in subsidiary matters, and do not necessitate that some of them cannot pray behind others. The scholars of the madhhabs themselves permit praying behind those who hold differing views. It is permissible for a worshipper to pray behind an Imam who is not of his madhhab, while maintaining the actions and sayings that he considers pillars or obligations, as long as this does not lead to a disruption in following the Imam. So, if the worshipper is Shafi'i, it does not harm him to sit briefly to send prayers upon the Prophet, peace and blessings be upon him, if the Imam has already given the salutation.