Nitrokey Pro: How many keys? - "Best practice" for Mix GnuPG and S/MIME?

As mentioned in: … an-i-store "…can store three RSA key pairs. All keys use the same identity but are used for different purposes: authentication, encryption and signing.

My situation:
I use both, GnuPG and S/MIME keys. I want add GnuPG and S/MIME keys to my Nitrokey Pro.

Referring to linked documentation above and assuming I started with GNUPG sub-keys - one for authentication, one for encryption and one for signing - I’m wondering, if there is still space for my S/MIME client keys?

Or, do I have to make a selection? Or forget about GnuPG sub-keys for authentication, for encryption and for signing, but one GnuPG sub-key for everything in order to have enough space for my S/MIME client keys?

What would you do in my shoes?

cheers & thx for your feedbacks

FYI - Maybe related as well:

The same RSA keys are used for GnuPG/OpenPGP and S/MIME/X.509. Basically you should generate the OpenPGP keys first and afterwards generate a X.509 certificate for one of the keys. There is space reserved for one X.509 certificate. So you would end up with 3 x RSA keys and 1 x X.509 certificate.