Testing multiple placements under one Ad Set is now posible. AdRules offers a tool to help you customize your assets to fit the requirements of each placement when you select specific placements under Edit Placement in Adbuilder. You can refer to those here.
Selecting the appropriate creative for each placement maximizes conversion rate and ROAS.
From AdBuilder, in Placement, choose Edit Placement. There's an option you can select to support only those placements that support asset customization.
From AdBuilder, in Creative, you can resize an existing creative or select a new image for each placement.