Best left to modders. The two sides are designed to fight eachother, and I don't think aliens vs aliens would work well. Plus marines don't have head specific hitboxes, so no headshots in marines vs marines.
Marine v Marines could work, set up one set of models to look "piratey". But the TTK (time to kill) would be rediculously short, and exos would be worthless even against incompetent enemies due to how fast they would die to lmg fire (a carapce onos has 3300 effective health, a armor 3 exo only has 1160 effective health). I mean a marine can down alien life forms pretfy damn fast if they are actulay hitting (with the exeption of the onos which takes several marines), now compair size and speed of mariens vs aliens and it will be much easier to get hits in on marines.
Alien V alien would be realy hard to get a solid IFF going, by which I mean how to tell freindly skulks from enemy skulks. Trying to creat an easily identifiable alternate colorscheme that doesnt clash horrendisly will be dificult. It would be entertaining to watch skulk v skulk battles though. Each life form would be obscenely stronger than the last. Lerk gas and spikes and flight vs skulks, jetpack lmg is already anoying and they can only do super jumps, lerks full on fly. Fade and onos would be crazy, aliens dont have the dps of marines, and the mellee nature means its much harder to focus fire compared to ranged presure. you could never catch a fade in order to kill it with mellee and lerk spikes dont hit nearly hard enouph to not let the fade blink away and heal. And onos, well they would take longer to bite down than a power node, and all the while they will be smacking you with thier horn, lerk spikes and gorge spit dont even do close to enouph damage to deal with them and everything else would need to get to mellee, where the onos deals more damage with a wider hit range than they do.
TL:DR marine v marine could work but it would be brutaly deadly, Alien v alien would however be horidly ballanced.
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Alien V alien would be realy hard to get a solid IFF going, by which I mean how to tell freindly skulks from enemy skulks. Trying to creat an easily identifiable alternate colorscheme that doesnt clash horrendisly will be dificult. It would be entertaining to watch skulk v skulk battles though. Each life form would be obscenely stronger than the last. Lerk gas and spikes and flight vs skulks, jetpack lmg is already anoying and they can only do super jumps, lerks full on fly. Fade and onos would be crazy, aliens dont have the dps of marines, and the mellee nature means its much harder to focus fire compared to ranged presure. you could never catch a fade in order to kill it with mellee and lerk spikes dont hit nearly hard enouph to not let the fade blink away and heal. And onos, well they would take longer to bite down than a power node, and all the while they will be smacking you with thier horn, lerk spikes and gorge spit dont even do close to enouph damage to deal with them and everything else would need to get to mellee, where the onos deals more damage with a wider hit range than they do.
TL:DR marine v marine could work but it would be brutaly deadly, Alien v alien would however be horidly ballanced.