You dad needs to f**king grow a pair! He seems to boss you around with everything! 
 
Anywho, yes, you can use different sized rotors, just renember the size on the back you have to get a +20mm adaptor than what you would have on the front. 
 
So if you had a 160mm rotor and used it on the front, but then you decide to use it on the rear, you would need to get a new rear adaptor+20 (180) 
 
AMIRITE?