If my highest tank BR is 9.3 and I have the phantom should I put it in my line up or will I cause myself to always be up tired. Never really understood the BR system anyway. Haha thank you, have a good day and say flying!

You will be just like 90% of US players at top tier, with their XM1 and Peten lineups.

You should probably wait until you have atleast one Abrams

Day*

No, you shouldn't. But you will be able to use it anyway.

In RB you will be uptiered to 10.3, in AB you wont as it will ignore the plane in the line up. Depending on what the 9.3 is you could probably find a way to hold your own and get enough SP to spawn in it regardless.