I2C -internal and external pull up
It depends on the specific architecture you are using.  If the microcontroller you are targeting has internal pull-ups, then you need to make sure that they are strong enough should you enable them to be able to omit the external pull-ups.

If it isn't strong enough, then you don't need to enable it and you'll be fine with just using external pull-ups.  

If by some freaky architecture, the pull-up is always enabled, you can effectively treat the internal pull-up as a resistor in parallel with the external pull-up since they'll both be pulling up to VCC.  Realize that when they are in parallel, the resulting resistance may be too low, so make sure that the end result is still capable of being a proper pull-up.