Are we allowed to curse non-Muslims by praying that they be doomed in this world and in the Hereafter. Our imam at the local mosque often recites the Qunūt-i-Nāzilah in the Fajr prayers which is really harsh on the Kuffār. They are condemned in this prayer and really taken to task.
Before I answer your question, I would like to point out a fundamental premise in understanding various directives of Islam: With the termination of the institution of Risālah, there are certain directives which cannot be applied in a period which is devoid of Rusul. The basic reason is that in the period of Rusul:
a. The fact that the truth has been revealed in its ultimate form can be ascertained.
b. People who have knowingly denied the truth revealed by a Rasūl can be pinpointed.
Today, no Muslim preacher is in a position to reveal the truth in a manner a Rasūl is able to nor can he pinpoint who among his addressees is guilty of knowingly denying the truth. Consequently, all such directives as relate to people who knowingly deny the basic truths about which it was certain that they had been delivered in their ultimate form are not related to this second period of history, which is without such personalities as Rasūl.
Cursing non-Muslims or expressing ill-intention towards them is one such directive. It is only allowed if the two above mentioned conditions are fulfilled. Today, since a Muslim preacher can neither ascertain that he has revealed the ultimate truth nor can he pinpoint people who have denied the truth, he must continue his efforts of propagation all his life, and remain a well-wisher of all the non-Muslims of the world. Therefore, as far as the Qunūt-i-Nāzilah is concerned, it was only the Prophet (sws) and his companions who had the prerogative to read it. We later Muslims do not have this right.