XOps has an expanded gauss armory mod that has that gauss sniper and autocanon you want.
-Changing gauss weapons to be actual rail guns and thus AP damage leaves the old 'gauss beam' damage to be changed into something else to be used by a new set of weapons. Although "gauss weapons" can legitimately refer to a charged particle beam as well. Lore-wise, I have no idea which particular version would work well under water.
-Stun rounds for the dart gun and riffle seem like great ideas, especially if someone's going to the trouble of re-skinning them.
I know that, Blank. I always do UFO on OXC, not TFTD (Well i haven't tried some TFTD mods). When i do my own OXC storyline and mods with probably someone who is not from openxcom.org, openxcom.com and the OXC forum or a big or small band of role players/RPers at June, July, August, September, October, November or December, i'm gonna do and make my style of everything whatever i want to unless i give credit to anyone who makes mods, designs (Of weapons, Of craft weapons, Of items etc.), rulesets, ruleset references and other stuff.
But. I'll be working on an alien race more similar to the Zerg from Starcraft, the Scrin from C&C, the Tyranids from Warhammer, the Reapers from Mass Effect (I liked to play ME, but no), the Black Arms from the Sonic the Hedgehog franchise (Or the Shadow the Hedgehog game?) and other alien races which will be a threat to X-COM, the UFO:EU/TFTD aliens and X-COM Apocalypse aliens and they are more stronger than them.
I guess i'll try some TFTD mods very soon.
Also look for the IonWeapon mod https://openxcom.org/forum/index.php/topic,4205.0.html
Fwiw, in my personal mod i changed the Gas Cannon into a Grenade Launcher and added a Stun-Grenade ammo for it. This could be an alternative for your dart-rifle stun ammo idea. Still, a stun ammo for the dart weapons could make them more useful, yes. But be careful and don't give it too much damage or recheck the aliens vulnerabilities, the game can become way too easy by doing so. Been there...
I see. That would look pretty good. But changing the Gas Cannon into a grenade launcher sounds nice.
Besides, shinr is not online.