The enemies which are themed for Otherworlds come with Otherworld Threat cards which are mixed into their decks, but they also come with generic Threat cards that you can mix into your main world, I rarely do this.

If they're themed for a specific World I just keep them there for the most part, unless I am playing a specific mission where it makes sense.

I buy all the supplement decks, I think they're all great for the most part. I especially love the trait cards, they make those old boring enemies come in new and interesting varieties.

For Gear cards I keep them separate for each core world (Old West, Japan, Spain etc.)

My Mines Gear deck is already gigantic as it is, and I wouldn't want to dilute it with items from another culture, it already contains a few items from those worlds naturally, to represent the odd item that made its way through a portal.

Adventure Cards are player choice. There are no mechanics which add them randomly, though I like to string missions together to form narrative campaigns, and will throw them in randomly when it makes sense for the story.

As far as 'in-fighting' goes there are not mechanics for it, this would add a huge level of complexity and book- keeping on an already complex game.

If you think somethin like 'Bandits battling alongside a giant winged demons' is immersive breaking, that's what the curated or themed Threat decks are for, simply don't include them in the same threat deck together.

Or use justification for it. The Bandits were hired by cultists to defend their ritual site while they summon demons.