Combat Tentacles: Well, I believe I mentioned the dar-cúlno already. The uplifted octopodes specialize in this sort of thing, combat exoskeletons and all.
They’re also quite common as combination manipulators, motivators, and doers of violence on combat robots used both underwater and in microgravity.
Multi-Armed and Dangerous: Well, I’ve mentioned the Eight-Legged Legion of octopus uplifts before, for a start, but even the people with humanoid morphology have received the ambidexterity and multitasking upgrades required to at least dual-wield. (A lot of the spacer clades have switched from the two-arms-two-legs model to the four-arms model, but the need to hang on to something to avoid action-reaction problems when shooting don’t make this as useful as it sounds for this purpose.
It’s still largely averted beyond that and the other naturally multi-armed species, though, because while you can – and some people do – add extra arms over and above one’s natural endowment, you can get much the same effect with even more flexibility by slaving a bunch of quasi-autonomous battle-drones to your neural interface, or occupying a swarm-bodied cybershell, which will not only let you have plenty of “arms” to fight with – just not restricted by being attached to you at one end – it’ll let you surround the enemy personally, too.
Everything’s Squishier With Cephalopods: While also one of the more difficult candidates – because their biology is so weird – they were one of the primary candidates for uplift, because they’re already remarkably intelligent, they’re alien enough to have a really interesting point of view, and an underwater native species with such nimble manipulators would – and did, as it turned out – be really useful with the ongoing colonization of the oceans.
And, for that matter, space. Modified octopodes adapt really well to a microgravity environment.
(Oh, and you know those little sub-brains for each tentacle? They mean that the dar-culno, the uplifted octopodes, are one of the very few species in the Galaxy that can dual-wield – or rather, hextuple-wield firearms and hit six targets at once. Don’t fuck with the Eight-Legged Legion, m’kay?)