Yeah, a trombonist is a important to a Ska band but the trumpet is great in any all around bands and is a great way to test you skill.
From what I have seen from many Ska bands, the most common types of horn sections are (the number of instruments isn't that important in the long run):
Saxophone and Trombone.
Less Than Jake
The Mighty Mighty Bosstones
Trumpet and Trombone:
Reel Big Fish
Forces of Evil (same thing but whatever)
Trumpet, Saxophone, Trombone:
The Hippos
Bad Manners
Catch 22 (new)
Madness
Like I said, the Trombone is a big thing In most ska bands while it seems to be either sax or trumpet or both. I personally see the trumpet to be a great instrument all around in and type of music while the trombone is important in ska. Just my opinion in this. I hope you enjoy whatever instrument you pick, I play Alto Saxophone and love it.