Filtering capacitors and inductors on power supply, how to choose and size?
1) Neither, this C - L -C structure is a low pass filter, it blocks high frequency voltage variations from +15V_ISOL to reach the chip.

2) You could calculate a bandwidth for the filter consisting of 6.8 uH and 11.1 uF (the sum of all capacitors) formula:  Fc = 1/(2piSQRT(LC)) = 18.4 kHz
So at 18.4 kHz a signal would roughly be halved in amplitude.
How much the most left 10 uF cap filters depends on the impedance of the +15V line.

3) That depends on the expected frequency components on the +15V line, how sensitive the IC is for certain frequencies (it might supress low frequencies already by itself but might not be able to supress high frequencies). Available board space, cost of components, etc...

4) Several in parallel because a 10 uF cap is usually not so effective at higher frequencies, this is where the 0.1 uF comes in, it is more suited for high frequencies. Look at a datasheet of any capacitor and you will see it will only behave as a capacitor within a certain frequency range. By combining several caps, the effective frequency range is extended (the combination gives a better capacitor). The 10 uF at the left also gives some filtering.

5) for small value caps like 0.1uF a 25 V version might not be available or a 50 V version was already used elsewhere in the design. As long as the voltage rating is high enough any rating can be used. The choice depends on price, availability etc.