Out of the box, extensions starting with 1 and 2 are fine, but 3-9 are reserved for features... It is fairly easy to rework the system to clear up 5, 6, and 7, but 3 are the default feature codes, and 4 is reverse transfer (you really want this to be a single digit, trust me), 8 is reserved for outgoing access and 9 for 911 (don't mess with that).
The reason you may have been told not to use 2 is most "system" things like hunt groups, CRA's, etc. use extensions starting with 2 by default, but this can be changed as well.
Networked systems tend to present challenges for the 250, but this really depends on your system's size and number of systems to be networked. Can you give us a scope of the overall project?
Also remember that really only phone extensions need to be unique... feature access codes, and many other things can be in duplicated in multiple sites depending on how you implementing them (although I don't recommend this if possible).