It's really weird though, as: 1. He is immune himself 2. He knew Master Chief was with the librarian. Either way, me not knowing what the word meant procured how the Composer works form my mind.

yeah, that line struck me as odd myself, especially after the terminals stated that he himself is immune. I still don't think the line implies that the composer is somehow partially organic in nature, although that is certainly possible. Perhaps the Didact is not immune to the effects of the composer, but merely is too screwed up biologically to be made into a Promethean knight like his soldiers and the humans. I don't remember quite how he worded that line in the terminal, so I don't know if that possibility exists. Also, it's possible that it was just an excuse to get out of being composed himself, but I can't see the Didact being that cowardly, however evil/insane he had become by that time, so I doubt it.

Well, not biological per se, but it mimicks biological pathogens. See "Energy Virus"

It's certainly possible, I wont deny that it is well within the forerunners considerable technological prowess, but I'm just saying that the inoculation line does not necessarily imply that.