Thanks for the kind words and advice. While yeah, zerg don't make sense too much as literally having items per se, we gotta recognize a few things.
0. Thank you very much for pointing out the likes fighting and possibly no fear thing makes it so that your non-military units won't run from fights. I'm working on a mod just for my own fun that will make the game way less micromanagey
1. While this makes zerg a bit more hermit-crabby than usual, having integrated carapaces/weapons may not be desirable. After all, if I'm not mistaken, an actual carapace/shell, once its broken, won't heal without medical treatment, right? We absolutely want to avoid that level of micromanagement, at least for the zerg. Since armor seems to increase in thickness with the wearer size for multicaste civilizations, bone armor (probably the most typical example) obviously won't do much for zerglings, but would contribute to making the huge ultralisks extra durable, as is appropriate. Utterly sacrificial zerg units, probably created by special abilities, can definitely have natural weapons/armor tho.
2. We could call them "evolution enzymes" or "external symbionts" or somethingorother.
3. In the original game, zerg and terrans alike research weapon/armor upgrades in the same way, which suggests that both terrans and zerg could get masterwork weapons, armor, etc.
4. Zerg tearing apart dead terrans and protoss for meat, bones and leather feels appropriate. If not that, at least using dead bodies to create infested terrans, sunken/spore colonies, changelings, zerglings, or broodlings would be really damn cool. Starcraft 2, post-HoS, has at least four zerg units that create free other zerg units, to really help create that 'limitless swarm' feel.
As to other races...
There are tissue layers, such as skin, muscle, and fat that can't be "broken," correct? I'm thinking that perhaps even terrans would have very different body layouts, because while knocking out the teeth of a power armored marine is not important, puncturing his suit may be; instead of lungs causing difficulty breathing, it may be their air recycler system part (rendered as an organ). Terran medics may have the ability to heal their friends, but (I'm not sure) such interactions can't be used to repair broken/severed tissues, so you'd have to get visited by a "suit repair" (bone doctor) type or eventually succumb to infection (zerg microbes in the atmosphere?). Its also fitting that, since the least terran unit started off encased in metal, that perhaps they have 'natural' metal tissues to protect them that they can't really remove (nude terrans running around on Tarsonis Orbital makes little sense). So wearable armor-type items for terrans would probably represent add-ons to their suit.
For protoss, as they can apparently run around in space completely exposed, this is less important. Also, the fluff for stalkers, dragoons and immortals suggests that they can and do take critical levels of irreparable damage; so protoss are perhaps the only types that being able to receive permanent damage fits. They could, however, upgrade a normal type into a stalker, dragoon or immortal to ignore that damage, or even there may be an interaction that raises the dead and transforms them into dragoons. Also, hopefully there's a way to mod in a good force field tissue that won't run the risk of infection.
Just some ideas.
Also, protoss have the judicator (noble), templar (warrior) and khala (worker) castes.
EDIT: Re: vespene, we might be able to create some system replacing magma's role.
I have heard talk of teleportation and summoning being possible, so I'm curious as to whether some possibility of nonstandard sieges (protoss teleporting into your base, dropships airlifting troops into your base) would be possible. Idea being not so much to punish you for playing zerg, but that the zerg should be more about trying to rapidly bring the swarm to respond to intruders rather than creating cold, reliable deathtrap hallways. Mechanisms really don't fit the zerg motif in general.