I have a mod that modifies behaviour depending on a keyword attached to a piece of clothing. I'd like to attach this keyword to other clothing in another module. The load order is this:

skyrim.esm

.

.

clothing mod

.

.

keyword mod

as ordered by BOSS. Now, if I renumber the keyword to inject it into skyrim.esm, can I use it in the clothing mod without problems? Even if it does work, Is this considered to be good practice or not?

Thanks.

Yes, you would be able to do this, so long as BOTH mods inject the same keyword into the file.

You will be better off doing so as an injection into Update.esm though. The CK will whine and cry at you about using Skyrim.esm for that, but oddly has no qualms about doing it to Update.esm instead.

Good, thanks. I would need to add update.esm to each mod as a master though, correct?

Yes, that's correct.