Is it permissible to pray with someone who is performing a Sunnah prayer with the intention of being an Imam for a person performing a Fard (obligatory) prayer, such that the presumed Imam says the Taslim (salutation of peace ending the prayer) and the follower then stands to complete his prayer, and what is the evidence for this?

The Shafi'is and some Hanbalis permitted a person performing a fard (obligatory) prayer to be led by someone performing a nafl (supererogatory) prayer. They cited as evidence the hadith of Mu'adh ibn Jabal, may Allah be pleased with him, who used to pray with the Prophet, peace and blessings be upon him, and then return to lead his people in prayer. Therefore, there is no harm in doing so.