Since I have been mentioned here Yes, Floss-Shop is a small side business of Petra and me in Germany. We are doing this since 1999 and have good relationships with many free software projects and help them with our shop, like selling merchandise and giving back a contribution. That’s why we came to sell the OpenPGP cards, I know Werner Koch for many years and help decoupling the card sales from the free software project GNUPG.
That being said, in general yes, the OpenPGP card, regardles where purchased, will work with the Librem5, BUT … you need it as micro-SIM format, which FLOSS-Shop can not offer (only ID-000). So I would recommend, if you want to use an OpenPGP card in the Librem5, to order it from Purism with the phone - it is the same card as the one from FLOSS-Shop just in the right form factor (and a nice Purism logo on it
The OpenPGP card supports three RSA keys up to 4k, right. Usually two are used for typical PGP use, one encryption and one signature key. The card also allows to store an X.509 certificate. Starting with V3 of the cards these also support elliptic curve cryptography (ECC), up to V3.3 only the NIST and Brainpool curves are supported. As far as I know starting from V3.4 also Curve 25519 is supported, which is supposedly more secure than the NIST or Brainpool curves, but I have to double this with Werner and also when we can start getting the V3.4 cards. ECC is interesting because it is much faster than RSA. Signing with a 4k RSA key can take a really long time on the small smartcard processor.
For full OpenPGP card specs please see: