Is it permissible for a Muslim to make supplications (du'a) that are not from the Sunnah after completing the remembrances (dhikr) following prayer while still in the mosque? And is it permissible to specify a certain number for them? And is it permissible to specify a particular formula of glorification (tasbih) for each of the five daily prayers?

It is permissible for the worshiper to remember Allah and supplicate to Him with whatever he wishes after the post-prayer remembrances. However, specifying supplications and remembrances with a certain number or allocating a specific formula for each prayer is not permissible, because the fundamental principle in acts of worship is tawqif (that they must be based on divine text and not personal opinion). What has been reported in various forms should be performed sometimes this way and sometimes that way, and a Muslim should not restrict himself to a single form.