Is it permissible to pray behind an imam wearing clothes with a cross or an infidel symbol, or next to a person wearing them? And is the prayer valid if performed in such clothes, knowing that wearing them is forbidden?

Prayer is not permissible in clothes that contain forbidden images, and it is not permissible for a Muslim to wear them, whether in prayer or outside of it. The prayer of one who prays in them is valid, though sinful, and there is no harm in the prayer of one who is beside him.