That was my order from Mouser. You don't need the last item. That's the FET you already installed for the fan.
You need 1x PCA9533 and 1x 74HC3G14DC and 3x of the FDV301N FETs, but I ordered a few just in case I messed something up. You will also need 3x 100K resistors and 3x 1M resistors and a 4 pin header
The resistors appear to be 0603 packages (haven't confirmed)
24V RGB LED strips are not so common. You could find some 12V LEDs and change out the resistors. Just double whatever resistance they are to keep the same current.
I have attached the schematics and bill of materials for the mightyboard as well.