There is nothing wrong with one who is offering an obligatory prayer to pray behind one who is offering an optional prayer. Also, there is nothing wrong with a person starting his prayer alone and then becoming an Imam when someone else joins him.

Anticipating or preceding the Imam in prayer is forbidden, and if it is done deliberately, it invalidates the prayer. Delaying following him, like preceding him, is not allowed.

If the Imam’s mistakes do not alter the meaning of Al-Fatihah, there is nothing wrong with praying behind him. But if his mistake changes the meaning, then people should not pray behind him if he does not benefit from prompting.