1. UV:
It’s very important for each model to have a UV with two rules applied: first one is all of the UV islands need to be on the main square, and secondly there is no overlap allowed.
You have a couple methods to achieve this, the best one is this (this one has some other advantages too):
Create 2 UV layers (UV channels). Layer0 is the UV for the textures and it doesn’t matter how you map the UV, use whichever method you’re used to. Layer1 is the UV used for light maps, this one needs to have those conditions applied. All 3d packages I’ve seen, have a simple UV mapping method that does this automatically for you (All UV islands in the main square, with a 1% gap between them for example), so you probably just need to use it in a new layer.