I just got my first shotgun, and as you can guess it's a Remington 870. All black 870 super magnum with a 26 inch barrel. Bass pro shops had a killer deal on Black Friday weekend

So I have a couple questions, and I'm hoping you guys can point me in the right direction
1.) i'm only familiar with handguns, I have a Glock and a Smith 357. The sales person was explaining a break in procedure to me for the shotgun, and stated that I should put 50 to 100 rounds through it before cleaning. Are there links to existing break and procedures on the site?
2.) I want this going to be kind of a jack of all trades. So like most people I want to put a 18 inch or 18 1/2 inch barrel on the gun. Why is it that with the 18 inch or 18 1/2 inch barrel I can only shoot 3 inch shells?
Thanks in advance for all the help