I got a question. When i finish a vehicle model in XSI export it and make it a msh. Do i got to do the Nimlot’s Bconstructor part described in the basic modeling tutorial? Is it any other things in the tutorial which i dont need to do?

If you are using XSI real versions (not modtools) and exporting the msh with the Pandemic exporter then no you do not have to do anything with Bconstructor.
If you are using the modtools version you should be able to use Reps mshex as long as there are no collision primitives

What happens if I am using XSI 6.0? I mean, that's a full version but I think is not compatible with Pandemic addon, I am exporting some props for now, but what about vehicles?

It all depends, are there any moving parts like turrets and such? If not then you would just need
to add weapon hardpoints nulls (like hp_fire) and it should export with mshex. If there are moving parts then you would need to find someone with version 5.11 or earlier to export it for you usign the pandemic exporter.

as for the first scenerio they only thing to do is to try and see.