Adding in custom instruments

You can create new ones in a custom instruments.xml file (look at the built-in one for an example) and add it as 2nd file in your preferences.

If you'll write more regularly for such an ensemble, you might not even bother with defining them all in programmer mumbo-jumbo:
1. Start by creating a new score using the default instruments.
2. Then, for each instrument/staff, use the staff properties to set the desired transposition
3. Save your score into your templates folder

Next time you can select that template from within the new score wizard and all those settings should be preserved.

Are you using the treble clef versions of those instruments? If so, the clef and transposition should already be correct. If you are experiencing otherwise, could you explain in more detail? Or maybe you just aren't finding the treble clef version? Use the search bar at the bottom of the instruments window to find them; they won't be listed by default.

BTW, manually changing the clef and transposition isn't particularly difficult either, and you should only need to do it once. Get it right, then save the result to your Templates folder, then just always use that when creating new scores, as mentioned above. And this also applies even if you do use the built-in treble clef versions. Set up all the instruments, then save as Template so you never need to mess with that again.

All of the instruments you named already exist. In the instruments dialog box (that opens when you press i) search for the instrument name. For example, when you type "trombone" you get

which includes the treble clef trombone.