Why do you add interest expense back onto CFO to get FCFF? FCFF to me sounds like it should just be CFO-capex?

FCFF is the cash flow available to all the stakeholders of the firm - which includes both debt and equity shareholders. CFO is calculated starting with NI - in which interest expense is already deducted. So you need to add that back. If you calculate FCFE (which is only free cash flow for the equity shareholders), the interest expense has been deducted already, hence no adjustment is further required.