Are Samsonite gears compatible with modern Technic gears? Do the old Samsonite gears (above) work with current Technic gears (below) 



LEGO bricks tend to be backward compatible with older bricks, but since I don't own any Samsonite gears I cannot tell.

Image credits belong to Technicopedia.
They are not compatible, the teeth are completely different.

Notice that the red old gear has 9 teeth and is the same size1 as a current 24-teeth, as illustrated by below:



The axle hole however is compatible, so you could have a construction using both types.

1. Actually, the distance between axles to have two of these red gears mesh is the same as the one required for two 24-teeth ones (60 ldu).