SSH v1 is deemed insecure and is widely abandoned anyway
Libssh2 supports the different layers
Gcrypt or OpenSSL Knownhost works with OpenSSH style knownhost files
See how its almost the same, just a little extra cruft added for SFTP instead of SCP.
An additional abstraction layer. Most useful if you deal with more than one protocol, or perhaps if you want to work easily with many parallell transfers with ease.
SCP speed is on par with OpenSSH SFTP speed is lesser than OpenSSH due to protocol complexity and how the libssh2 API works and the work needed to fix it. Faster than libssh still Features wise, it offers a lot. Bug wise it is fairly stable and solid, even if there still are bugs appearing that need to be fixed. Several existing commercial applications are relying on libssh2.