I havnt yet made the full conversion to gunslinging (though I'm gradually building up a bigger arsenal) and I only ever bomb in LD, so just bear that in mind, but ill give what advice I can. Polaris is awesome, and from my experience with it Biohazard is incredibly fun to use. nothing's quite as satisfying as loading up charges on a giant lichen colony and blowing it to smithereens :P I would highly recommend it as your Shadow gun, though it can be a little awkward to use sometimes. it takes a little getting used to. if you find you don't like it Umbra Driver might be for you. I'm not a big fan of alchemers personally, I find the 2 shot clip annoying. Bear in mind I'm pretty biased so take what I say with a grain of salt :P

I have no experience at all with Calahan and my only experience with the needle lines has been the 3*. I find it fun to use and don't have much of a problem with the standing still to shoot thing. Calahan's the same anyway, albeit slightly shorter (or so I've heard, I'll leave that for people who know more than I do to confirm). Blitz would be better for FSC and Vanaduke though if that's important to you, and it outdamages just about everything.

Argent Peacemaker is generally believed to only be useful at FSC and apparently only has limited usage elsewhere. the split damage kinda sucks. would be good for LD though ;)

As for armour I really don't know though just from looking at the Wiki I'd say Nameless is the best all round, use Justifier for piercing strata. get 2 damage trinkets and you'd be set. CTR is pretty much useless on anything but Blitz... on that note, with Max ASI and CTR that thing would be a monster... just a thought. Trying to get a CTR UV (Med at least, Very High if possible) on it would probably be the best idea.

"Also, is the Iron Slug crap?"

Yes yes a thousand times yes the only reason you would want one is if you really like the pop shot play-style of the callihan but want to be able to take out jellies and constructs. #1 normal type damage is bad you want to be maximizing your damage not leveling it out the only normal type gun i could recommend is valiance because of its guarantied knock-back it is a great survivability tool. The interrupt is useless against jellies as they cant be interrupted and most constructs have a very small interrupt frame so it is a very bad choice. Your polaris (while I personally dislike that gun) is more useful than any normal gun (excluding valiance). as another change to your arsenal get either a biohazard or a shadowtech alchemer instead of your silxersix. you already have an elemental gun and 2 piercing ones.