You demonstrate instruments as soon as you are done going trhough the handbook, have kids indicate preferences, and start testing.
What you lose in how far you go the first year, you make up in how much faster they understand blend and balance. By 8th grade, there wasn't a huge difference in how they started.
It's OK to try something different. You might even be able to pull some brownine points by taking one for the team this year in order to move forwrd faster.
My biggest sources of stress these days involve people trying to wedge square pegs in round holes becaause they always had square holes in the past, darn it!!