It would be more like a vertebrate with sections of armoured flesh and still having bones underneath, but yes you could make Zerg creatures.
About exoskeleton-endoskeleton combo: Couldn’t you also have something where an exoskeleton segment has ‘branches’ leading in that lead to an endoskeletal joint?
About odd limb numbers: What if you spend a long time on land, and evolve into a heavily asymetric creature, which may have an odd limb number?
I have to say, I don’t see why not, and it seems pretty damn badass. The only issue is that you’d have to change the skeleton up a bit. Our skeletons are filled with bone marrow and blood and stuff, while insects don’t have that at all. Their skeletons are a bit more disposable.
You might be able to create something for balance purposes. Just extend your spine or something, and create a long, slim limb you could then use to help your creature in balance. Perhaps you could even make it be able to pick stuff up.
What would the purpose of having an exo - endoskeleton combo be?
I’m guessing you get the perks of hard to crush and to penetrate, but having two sets of bones would make your creature pretty heavy.
If it was a complete endoskeleton and complete exoskeleton, then if the endoskeleton is much stronger, then it could serve as a backup if the exoskeleton failed.