What is the ruling on one who does not pray Tarawih throughout the month of Ramadan without a legitimate excuse, and is he sinful for that? And is it permissible to break the fast at work with Shiites and Ismailis if the questioner is the only Sunni?

Tarawih prayer is a confirmed Sunnah. There is no sin upon one who omits it, but it is recommended for a Muslim not to abandon it. And whoever is unable to perform it in full should pray what is easy for him. As for breaking the fast with Shiites and Ismailis, if there is a benefit in it for reconciling their hearts and inviting them to the Sunnah, then it is permissible. But if there is no benefit in it, then it is better to avoid them for fear of discord and doubts.