Are you trying to get correct playback for this or  are you just concerned with how it looks on paper?

Both are interesting, but for the moment, as I’m in a hurry to send the parts, I would take the quick advice.
Thanks

For a quick solution with no playback, simply create your timpani staff as usual. Whenever you need him to switch to triangle, simply enter the triangle notes on a single pitch, perhaps the 3rd space E. Change the notehead shape to a triangle or x or whatever you want. You can do this with special tools or in the Utilities menu, under change>noteheads. Add a text expression stating that this is triangle and you're done. No need to create a percussion staff at all. You can change the clef to a percussion clef if desired, that's up to you. It will still be perfectly understandable to the player if you leave it in bass clef.

Yes I understand that’s the way I thought I would do it.
And do you know a solution with playback for next time ?
Thanks

I don't use percussion playback. Perhaps someone else will provide an answer to that question.