It is a known Halacha (the example below is from Kitzur Shulchan Oruch, 18) that you shouldn't mix words from different versions:
Everyone should pray according to his [accustomed] form [of prayer], whether it be Ashkenazic or Sefardic, or another, [for] they are all based on sacred origin. However do not intermingle words from one form with another, because the words of each form are numbered and counted, based on important mystical concepts, and nothing should be added or subtracted from them.
What often happens for nusach Sefard is you get a lot of optional words in brackets so the question is:
Must you either include all or exclude all or are there certain other combinations that "work" (and were carefully numbered and counted)?
What complicates this more is that Nusach Sefard siddurim often offer different sets of brackets (either unconditionally including or omitting certain parts).
For example, in Modim in can say "tzureinu, tzur hayeinu" where tzureinu is included, excluded or bracketed. Same with "rov oz" at the end of the last brocha. There are many other examples.