You have to draw that schematic better.
I hope this is better:
The red marked wires are + blue -
The expander pins A0,A1,A2 define the I2C adress, otherwise I could give them input with another 5V supply from the arduino (if thats possible)
And just to be 100% sure to connect the supply to the right arduino port, you mean this one:
Edit: Im still wondering if 2.5A is ok for my project because: 1 LED 21mA
42*21mA=0.882A 0.882A+Arduino current is still about 1 A I think, so does it matter that I have like 1.5A too much or do I need that much current?
it does no harm having a power supply that is capable of more current than you need. It is often a good idea to to run them at only 80% full capacity.
in my case it would be like 40% of full capacity is that ok?