You're free to do what you want, of course. But while "25C" is technically wrong, at least it's unambiguous in context. You're using "ID" in a nonstandard way (which is fine) but also in a way that's totally ambiguous (which is considerably less fine, at least IMHO).
You could similarly use "seatpost" for "seat tube" if you wanted to. But, like "ID," "seatpost" already means something. At best, that's an annoyingly quirky twist on terminology. At worst, people think you don't know what you're talking about when you mention that your seatpost has an ID of 31.6mm.
I'm not saying bike lingo is consistent, or even that it should be. I'm suggesting that you're making up new definitions for existing acronyms, which makes it harder to understand what you write. You can do that, of course, but then people like TheRookie and myself will call you out on it.
P.S.: I'm a mechanical engineer now, but before grad school I worked as a bike mechanic off and on for about fifteen years. In every place I worked, a mechanic who used "ID" to mean "internal distance" instead of "internal diameter" would have been viewed as a bit dim by the other mechanics. "ID" may be engineering speak, as you say, but it's also bike speak. (I'm not insulting your intelligence, FWIW).
Edit: fixed typo and added PS.