I have updated the schema, and I don"t have any errors anymore :
I have associated symbols with components (I just don’t find the arduino mega one)
When it comes to power, I calculated that it would be ok with the Arduino thanks to the resistors. (Not above 20mA per pin + not above 200mA overall) But I would like you to explain how to use a transistor because I never did… 
Can I set my PCB size ? Because I want it to be te size of my front plate of the FCU which is 256mmx82mm. (I don’t want to make a pcb as little as possible I just want to place the components where they have to be and that’s all)
Thanks for the help