The Mishna Berura writes that in the case of people leaving the minyan after it started, Kaddish should still be said after the Slichot. In the Halichot Shlomo it says that prayers written in Aramaic at the end of the Slichot, which should usually be said when a minyan is present, can still be said if six out of the ten people are still saying them. In that circumstance it is definitely permitted.