I am making a brass quintet, and I need two trumpets, a horn, a trombone, and a tuba. I have all except the trombone. Can a baritone substitute for the trombone part? Will it have a similar tone and/or style of play?
There will be differences in the tone quality, because the baritone/euphonium is a conical bore instrument whereas the trombone is a cylindrical bore. (Generally speaking, this means that the trombone will have a brighter tone than the baritone/euphonium.)
But in terms of range, the two instruments are pretty similar, and the baritone can certainly play the trombone part. Furthermore, the two players will typically read the same clefs, assuming they're from the same background. Just watch out for trombone-specific techniques that a baritone won't be able to play, like a long glissando.
And by the way, going the other way could be more difficult. If a baritone part has lots of 16th notes, it's probably not wise for a trombone player to attempt to play it (sorry, trombonists...).
Edit: As the comments helpfully state, there is a difference between baritone and euphonium, but I think it's safe to say OP isn't working with a British brass band.
As well as the answer about the tone quality of trombone vs baritone, you also consider the sheet music you play from.
Brass Quintet trombone parts are notated at concert pitch using a variety of clefs. Baritone players (in British Brass band at least) expect treble-clef Bb parts. So unless your baritone player is already a trombonist and used to the idea of clefs and transposition, you'll either have to write out some transposed parts