Is it permissible to take from used clothes distributed to the poor, based on the Almighty’s saying: "And those employed to collect [zakah] therefrom"?

You are not permitted to take from these clothes which you are entrusted to distribute, unless you are in need, because you are an agent for their owner, and an agent may not take from what he is entrusted with except with the permission of his principal. Furthermore, the share for those who work (in collecting and distributing alms) is in obligatory zakat (definition), whereas what you are entrusted with is voluntary charity. Moreover, the workers who are allowed to take from zakat are those appointed by the ruler, not a private agent.