Two different input in a buck stage?
Your selection of the SS26FL looks like a good choice for isolating the 5VUSB from the other +15V input power rail. This is a 1A Schottky diode that looks well suited to the application. This method of isolation is pretty standard and very simple to implement. 

The use of a Schottky diode is useful because its reduced voltage drop versus some other regular silicon diode allows a higher voltage to the voltage regulator in USB mode. This reduces power loss in the diode and will lend a small performance increase to the switching converter chip. 

I also looked at the data sheet for your LMZM23601V3 chip and see that it is also rated for 1A output so the above diode seems to be a good match as well since the average input current to the regulator will be less than the output current in this step down application. 

If you have not yet used TI's online switching voltage regulator design tool you should take a look at that. It can give you strong recommendations on proper capacitor types and values for your input and output caps.