السلام عليكم ورحمة الله وبركاته Are the terms 'ustadh' and 'sheikh' interchangeable when referring to my fiqh/Quran teacher? I heard that 'sheikh' should only be used for a big scholar (whether hes living e.g. Sheikh Al Fawzan, or dead), and ustadh should be used for a teacher/student of knowledge

Ustadh AbdulAziz al-Haqqan

Not really, alfawzan is an imam allamah, as for ustadh this is common in the west and some ustadhs when the people of knowledge refer to them they say sheikh so and so, and this is a form of respect then how about their students? They should also learn good manners and be respectful.