I have a customized Salsa Journeyman Claris. How small can I go for my small chainring? I'm not sure how to verify compatibility
The crank chainring bolt center diameter (diameter of a circle that passes through the center of the chainring bolts) is what limits the maximum and minimum chainring sizes.
It's likely that you will not be able to get a much smaller chainring on the FSA crank. To get to 24 tooth small rings you need an mountain bike crank. 'Two-piece' MTB cranks made for external bearings in threaded frames or press-fit bearings are not compatible with road frames as they require a wider bottom bracket shell. The FSA Tempo crank is three-piece for a tapered axle cassette type BB though, so you made have luck finding a three-piece MTB double crank with the chainring sizes you want. Bear in mind you will probably have to change the BB as different crank models require different BB axles lengths.
Check that your front derailleur can accommodate the difference between chainrings on the crank you select, and can be dropped far enough down the seatpost.