Q1: Yes I'm using the USB because I read the USB port can give a max of 500mA and If I and my datasheets are right, I'm only using 2 relays of 10mA each, and never use both at the same time. (+ the Arduino)
(I also have 2 tiny servos but theseone's barely moves)
Q2: The relay (or signal relay, as the DS says) is this one:
(5 volts of course)
With that relay I activate a power relay:
but I think that one dont interfere with the Arduino because is in a complete separate circuit powered with another source.
I'm not sure about the EMF, I read a little bit and I understood that the EMF is a voltage induced by the coil of the relay, I'm right? I need the diode with the kind of relays I'm using?
And as I said, sorry I can't apply this solutions instantly to my circuit, but I dont have access to my school lab every day (because my schedule).
P.S. the problem usually happen (but not every time) when the Arduino de-activated one Rele and activate the other. That's why my first idea was the delay of 100 between activate the second.