From one tuba player to another make sure you don't fall into a bad habit of using a mouthpiece that is too small. I started out with trombone and euphonium using smaller mouthpieces and upon some advice from Doug Elliott in the trombone forum (in general discussion about mouthpieces) started using a larger piece with great results. I actually have more range on the larger mouthpiece and am getting a much better sound. I've played tuba for almost 40 years. I've played trombone and euphonium for about 3. Had to remind myself to relax, play lots of long tones and enjoy! Best wishes to you in your new endeavor.
John 3:16
Conn Victor 5H Trombone
Yamaha 354 Trombone
Conn 15I Euphonium