Editing Squeeze and Jessie distribution ISO's

[Richard Owlett <rowlett@cloud85.net>]

I'm investigating creating customized installation DVD's &/or 
flash drives.
I have two use cases:
  * one *REQUIRES* Squeeze.
  * one could use Squeeze, but Jessie would have advantages.
  * neither has useful internet connectivity
My changes would be:
  * custom preseed.cfg [goal for one user would be no installer 
questions]
  * custom set of packages in pool directory.

My questions:
  1. the references I've found so far are years old.
     a. any recent pages?
     b. does it make a difference [possible side effects of 
adopting systemd]?
  2. Is there a practical limit on the ISO file size? It could 
be convenient to
     have all packages from the 13 DVD set of Jessie on the 
flash drive.