| Usually, From left to right facing the band (or Rhythm Section out) it's this: First Row Saxes: 1st Tenor, Lead Alto, 2nd Alto, 2nd Tenor, Bari. Second Row Bones: 2nd Trombone, Lead Bone, 3rd Bone, 4th Bone, Bass Bone. 3rd Row Trumpets: 2nd Trumpet, Lead Trumpet, 3rd Trumpet, 4th Trumpet, 5th Trumpet.
Sometimes there can be variations on this, but this seems to be the most common if you go see a big band. As far as what trumpet solos the much, I often see that in the lower trumpet parts. Usually the guys playing lead trumpet are the guys (or gals, of course) that are screaming out the high notes and aren't always the best soloists (that doesn't mean I've never heard a solo from the lead trumpet player, i.e. John Faddis, Maynard Furgeson, etc...), but lead trumpet players are usually trained just to play those high notes. Overall, however, I'd just go with whomever is your strongest soloists in all sections. Just because someone is sitting 4th bone doesn't mean you shouldn't let him solo just because he's sitting in that chair.
Hope this all helps. | 
