Does my reporting of employees' behavior to the manager, which resulted in their dismissal, count as cutting off their livelihoods, and what should I do about my pangs of conscience?

No one can cut off another's sustenance, for Allah is the Provider for all His creation. Allah Almighty said: "And there is no creature on earth but that upon Allah is its provision." It is also authentically narrated from the Prophet, peace and blessings be upon him, that he said: "Indeed, the Holy Spirit breathed into my heart that no soul will die until it completes its appointed term and receives its full sustenance. So, fear Allah and be moderate in seeking [your livelihood]."

Regarding your complaint to the manager: if you wronged them by saying things that were not true about them, then you have transgressed against them and wronged them, and you must repent and seek their forgiveness. However, if you merely clarified the situation of a negligent person after advising him, or complained about a brother's mistreatment of you, then there is no blame upon you.