Other IdPs can work -- SAML is a public
standard that anybody can implement -- and I've even added enhancements
to Domino's SAML SP implementation to fix issues that people have pointed
out on this and other forums. That said, only those two IdPs are officially
supported (and thoroughly documented), so any PMRs that you open to ask
support for help with any configuration issues that you might run into
with unsupported IdPs won't go very far.