There's a way to meaningfully play a tuba?
Ecstatic to play tuba? Then you should play tuba. But oh, wait, you're actually talking about playing Sousaphone.
Just play it. Geez, it's a pep band. The last pep band I played in, I played flute. This was great because (a) all the other flute players were female, and (b) the pep band sat behind one of the goals during hockey games (I should perhaps point out that the Sousaphone players sat in the back row and caught a lot of pucks in their bells; the flute players sat in the front row, right behind the goal, and were protected from flying pucks).
I'd offer only one bit of mostly ignorant advice: Don't use a mouthpiece that's excessively large. Getting a good mouthpiece (FOR YOU, never mind what anyone else says) will make a huge difference in your experience.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)