@rysiek@paolo pEp is implementing OpenPGP standard, so any pEp with OpenPGP client communication is compatible. pEp with pEp is additionally bringing extra features.
PGP inline is not used, bcs it's leaking too much metadata. So default is pEp, then OpenPGP and when no public key is available it is using plaintext. Idea is to make it easy for ordinary users, so all key management is done in the background. No need to manually import keys, decrypt/encrypt etc.