I’m building a small cmoy amp and my goal is to reduce it to SMD parts only, now I have a friend who repairs various electronic equipment and I asked if I could replace it with a lower voltage. He said yes along at its higher than the highest voltage on the power rail, anything above plus 20% would be a waste.
So here’s my question in your opinion could I replace 1.0 µF 100V metallized polyester capacitor with EEE-FC1V1R0AR . Also why would he choose a 100v Cap on an 18v max rail?

All info I have tried to put in the attached picture including the circuit

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From the creator of the amp at Tangentsoft :

C1 (film)
To understand the issues involved in deciding whether to add C1 or to put a jumper across this position, read my article Input Capacitors for Headphone Amps.
Many types of capacitors will fit here, up to 0.4" pin pitch. You will probably have to go with metallized polypropylene or polyester to fit a usefully high value cap here.
Optional? Yes, jumper across it.
Largest Part Size: 13mm × 6mm. Lead spacing 10mm.

I read the article but there was nothing about voltage