Yes any calls made from boost to boost, boost to nextel and boost to sprint are considered mobile to mobile and are free on the daily chat and text plan and on the monthly unlimited. However they are charged on the pay as you go.
No, it will give an error message asking for the unlock code. Once you get that error, you screwed up the sim and it will not work in either phone again and it locks up your phone and it cannot be...
Boost mobile phones and AT&T mobile phones are very similar. In fact, some of the phones are identical. While Boost definitely is cheaper for service, I can't say that their phones are any better.