If you plan to reload the gun you'll need a free hand, so use a light shield or buckler.
You'll probably want a good dex to shoot things with, likely enhanced further by your mutagen. Light armor might be better than medium. This could let you take a different archetype (siege gunner for int-based grit perhaps?) or stick with the base gunslinger, or even take a different class/archetype which gets proficiency with a gun.
Targeted bomb admixture should work. It does take a standard action to use and lasts very little time so you won't want to use it every combat.
Gun Tank gives you heavy armor and tower shield proficiency. If you're only going to Medium, it might not be worth it. Heavy armor isn't necessarily as limiting as you might think. Mithril full-plate has a max Dex of +3, and 4th level as a gun tank gets you Armor Training 1, for a max Dex of +4, which you're not going to outstrip unless you go past an 18 Dex (which you might, with your mutagen, but you need to weigh the amount of AC you're getting from heavy armor vs. the couple you might lose from Dex).
I don't see why Targeted Bomb Admixture wouldn't work, but depending on your GM they might get hung up on the 'create and throw' phrasing.
I'm playing a Warpriest/Gun Tank at the moment. One thing you may want to consider is Deft Shootist. You need Dodge and Mobility as prerequisites, but it prevents you from provoking attacks of opportunity when shooting or reloading a firearm. If you're heavily armored, there's a good chance you're going to end up in melee, and preventing those AoOs is probably worth doing. Since the range increment on a pistol is only 20', you're likely to want to be near melee at the very least, which puts you in range of people running up to threaten you. And you'll probably want Precise Bombs so you don't blow yourself up if you need to fire one of those explosive rounds at close range.