Getting all the optional chimera scenes is actually required to see the last extra one, so it's encouraged to not miss any - the point of no return for them also has plenty of warning with chances to save beforehand. With all of the chimera scenes, Iustirho's level falls to 10, but that doesn't make completing the game any harder - the levels (and Iustirho getting weaker / more tired) are mainly just there for presentation.