Yes! Some players wear shorts while playing softball. Just be careful about how you slide—you might want some extra coverage, like a slider on your knee.
I don't think it matters whether you fight Bowser before getting all the stars or not. As long as you have 120 stars when you defeat him, you'll get Luigi.
Spikeball is fairly simple and easy to learn, but can be difficult to master. Serious Spikeball players may eventually develop more elaborate strategies and nuanced skills that allow them to...