Oh, there is a misinterpretation around the 600 triangles thing. That is regarding a polycount article - and how the GPU reads models. What it said is basically that somewhere around 600 triangles (depending on GPU) it doesn't matter if you optimze more. So <600 triangle models are basically requiring the same amount of performance.
http://www.polycount.com/forum/showthread.php?t=50588
Performance-wise I think it's better to have one large texture, instead of several smaller ones. So in the instance of creating a room, it's better performance-wise to have all the room models in a 2048 texture rather than smaller textures. This has to do with something called 'drawcalls'.
However if the export of the room is made by bundling a lot of textures to one mesh I would rather recommend having them split up, for performance reasons.
Ultimately it's a matter of how you like to work. If you enjoy making the rooms as 'finished' models - do that. I find it nicer to split them into modular pieces, for the sake of 're-usage' later.