Should I use the same OpenPGP keys in certificates used to provision the YubiKey PIV slots?

I know PIV and OpenPGP are separate standards and independent applications in the YubiKey, but for newcomers like me they look very similar with their signing, encryption and authentication keys, use cases, etc.

After setting up my YubiKey with OpenPGP keys I’m wondering: is it advisable, useful and/or secure to load the PIV slots with certificates issued for the same keys used for OpenPGP?

Do i need to allow free space as an over provision on an Macbook Pro’s inbuilt SSD?

I have a 2017 15″ MBP with an in built 1TB SSD.

As i understand with SSDs the best practice is to leave a portion of them free as an “over provision”.

Do i need to do this the the SSD inbuilt to the MBP or if have Apple already accounted for this in giving me 1TB of space ? Eg. There is actually something like 1.1TB of space, but the firmware keeps that additional 0.1TB free as an over provision.