From a clarinettist. Small thing maybe but this is the straight scoop on how it really works.
I’d drop the D clarinet - don’t encourage composers to use that! Nobody has one, I’ve never heard of it in professional circles, and certainly not amateur. If you use it you’ll just have the clarinets asking you for a transposed part anyhow. Buffet’s cost $8k these days, and they don’t even make one I think, I did a search a while back and there’s only one or two manufacturers. I certainly won’t buy one for the vanity of “it has a better tone”. Uh, yeah, that’s what I’m here for is what they’re thinking.
Second is the A clarinets should all be alternate hand on the Bb’s, in other words have each (of 3) play both Bb and A. That’s the way it’s done all over the world. Every professional has both (and most amateurs) - I bought my first professional A shortly after the Bb. Having both as seperate players makes no sense, the key signature that’ll work in one will not be playable in the other
Third, Eb should really be an alternate/doubling too. Not used too much, but they way it’s done from San Francisco to Berlin is one of the regulars will specialize in it.
Fourth, bass clarinet is much more common, and there’s usually a dedicated player for it in most orchestras (who also alternates with Bb/A), but the bass gets more play time. I’d include that.
Fifth - three clarinets is less common, four is more common. A good starting point would be
1 . 1st clarinet Bb/A
2. 2nd clarinet Bb/A
3. 1st alternate, Bb/A/Bass clarinet
4. 2nd alternate, Bb/A/Eb clarinet
If you wanted to go further, you could include the saxophone. In really top orchestras they’ll call in an extra specialist, but it’s not too uncommon that one of the players will play it (I play both). It’s only used in Bolero AFAIK - a shame, a historical malfeasance that should be corrected. It would be neat if you included sax as a poke in the eye to get composers to use it more.
And that’s all you ever wanted to know about the modern clarinet section!