Thank you, everybody, for taking the time to answer my questions. For the first question, Todd's trick did what I needed. Now I am able to make seperate image files and put them together and make one big image. Thanks Todd.
I apreciate the answers to my second question too, but I am kind of lost. May be I am not good with layers and scenes yet, so I feel like I am not understanding something.
So, the first trick would be using 2D trees and 2D people and replacing them with the 3D trees and 3D people, right? What about the size (height) of the trees and people? Do I need to adjust that or replacing object will be the exact size of the orginal object. I could try it out myself and see, but just wanted to ask if somebody has already done it.
The other one is to make layers for those heavy components and turn off that layer while working with the image. But the problem is, at some point I have to be able to put them all together and show my boss. That is when I am going to have problem as I can not orbit around to show him.
Thanks for your time.