When you say 'small ATX case'; how many external 5.25 & 3.5 drive bays?
Any case that I have seen with less than 3 - 5.25 & 2 - 3.5 external drives have been for the (older) AT standard; consequently a 'full-size' ATX mobo won't fit the mobo tray and the available space.
The 3 & 4 external 5.25 cases that I am aware of have mounting hole patterns for 6omm fans only. You are go have to go much taller to even hope to find 120mm mountings.