Hello Cameron,
Unfortunately, they are not customizable. Take a look at this earlier forum post for an explanation of the different types. The values are used in Home Dashboard reports and other functionality that would be affected if you changed these values. Also, there isn't an interface to change the default value of the dropdown. On the other hand, I believe with some customization, you can make a new default value. I would have a 'Enter Value Into Field On Screen' action in the OnScreenLoad action to set the value of the Lead Status drop down to whatever value you need.
Greg