Thanks for your answers
Im sorry to bother you even more, but now I got 3 (hopefully final) questions:
Connect the ground to the Arduino ground and the +5V to the +5V power supply of your port expanders.
1. If I do so, I don't need transistors, right?
2. What kind of port do I need for this power supply and with what kind of port can i connect it to my circuit.
3. Is it possible to supply my arduino with it as well?
4. Would the external supply deliver 2.5A to each expander(Im not sure if the expander can handle this) and would this circuit be parallel as well?(else, how would it look like?)