I typically have all the instruments, or at least the main ones, decided before I ever start "writing".
My main setup these days is my own custom "keyboard" patches, bass, various ethnic drums to create a "kit" and on top of that, you can find basically anything. How I choose the sounds is dependent on what the piece is meant to represent.
I always see something in my mind. A picture, story, person or place that the tune is supposed to fit with, and that's how I choose what instruments to use. It's summer and I was thinking of Romantic beaches, and so that's why I'm using classical guitar and violin on this tune I'm currently working on.
This is a fantastic question and I was actually considering making a thread about it, but you've asked here.
The orchestra is easily the most-overrated ensemble there is. It is essentially born of this infantile obsession with making everything bigger and "more", but music written for it can effectively be realized with much smaller ensembles. There is also the issue that you lose so much soloist detail in such a large ensemble. Most of the time, I'd rather hear a nice, expressive solo violin than 18 violins playing.
The other issue with the orchestra is the time investment and effectively utilizing all of the players. I think this is why the orchestra has fallen out of favor even though everyone can write for it today.
I was having this discussion on VI-C last week with someone who was feeling burnt out with composing with orchestral samples. I really encourage people to write for smaller ensembles and bands more often. It frees up a lot of time ā no need to record tons of different lines playing the same thing or whatever ā it's more affordable, and having fewer instruments to work with will also help breed creativity, as limitations are known to do.