This is a great idea many teams do this including mine. Yes, it is very possible and legal to do this as long as you obey the Fix it windows. Building a prototype bot as we call it, is wonderful because it allows you to see mechanical design flaws, what the whole finished product is going to look like and possibly fix layout of electronics and pneumatics better by seeing areas were you have more or less space. Finally, this is a good way to have less experienced kids get acquainted with the robots systems like you stated as well.
Good luck,
Drew