H1B extension - how soon should I enter US to file for extension?

My H1B was approved in Mar 2012 and expires in Mar 2013.

First question:

I am planning to enter US in Oct 2012. Will I face any trouble because I am entering the country 6 months after the visa approval date?

Second question:

Is it mandatory for my spouse (H4) to enter US before H1B expiry date? Can my spouse enter US after I file for extension?