Level Design - Collision Detection and Triangle Count

Hello everyone

It's my first time with Unity and Level Design so I'm a bit struggling. I haven't attempted anything in Unity yet though. I'm a 3D modelling student that is trying to make a game off his own. But before I start modelling my assets I have a few questions that need to be answered. Thanks already for your interest and time!

I'm working on a game idea wich will have a level design style like the Scarecrow Dream levels in Batman: Arkham Asylum. Broken up and floating in the air.

Here's a screenshot and YouTube video's for the ones who haven't played the game. You'll have to skip ahaed in the video's btw. It are the parts like in the screenshot I'm refering too.

Video 1

Video 2

Now my question is: What's the best approach to moddeling the basic environment like the walls, floor, roofing for use in Unity? Do I make real large meshes or rather small seperate meshes to build up the environment with instances. By large I mean a hole section of a level in one mesh. And by small I mean for example a square piece of a floor and repeating that to make a hole corridor. Or something in between, the hole corridors floor as one mesh.

I've heard a lot of seperate meshes can make it heavy for the collision detection. The basic terrian is not an option for this kind of level design (I think) since the underground is going to be flat and not organic.

A better question is maybe this: Can I make a level with a lot of seperate meshes and high triangle count and then make one mesh of the hole level, very low triangle count, to handle the collision detection? I saw this in a making of from God of War some time ago.

Or are there better ways to handle this in Unity?

Oh, and if there are any tips on the max triangle count per level to run the game smoothly on a not too powerfull PC, that's always welcome too!

Sorry if it basic stuff I'm asking or this is clearly explained already by Unity or here on UnityAnswers. I don't have much time at the moment to do much reaserch but I have to start modelling soon.

Again, thanks already for your interest and time and hopfully also for your answers!

Luch!