Yes you can. In Florida in 2011 we had U10 rookie tournaments that were quickstart and and at the same time all other higher level U10 were regulation yellow ball/racquet length/court size. It was not confusing. It just made parents upset because they knew that in 2012 this option of regulation U10 would be gone because of the mandate. Most of the parents who are upset do not dislike QS. Most believe that if done correctly it can be great for the development of young children (ages 8 and under). But to mandate that all U10 be QS is just too much.
With all the recent changes I have come to believe that the USTA has it's hidden agenda and it's not to reduce travel or limit the number of missed school days. They are under the gun to produce the next American champion and it will try any idea to see if anything works. Limit draw sizes, increase wild cards, if any junior shows promise latch onto him/her and proclaim your greatness.
The USTA is the governing body of American tennis. It is a highly profitable monolith. What it says, goes. If you don't like it, too bad. Hopefully, after a few years it will see the errors of it's ways. But unfortunately a good number of juniors will have been screwed in the meantime.
They don't care who they mess up..... This is so sad.