Hey Christine,
With engagement programs (unless something has been changed), you cannot create an email program child within the engagement program. That takes that option and the idea of "selecting the champion and sending to the rest of the cast" off the table.
The other two options, doing a smart campaign 50/50 send or right click on an email to add a test will both send the champion/challenger at the same time. After you see the results, you could choose a champion for future casts.
I'm not sure how your EP's are set up, but if they're set up within programs, then you must do the 50/50 smart campaign as the right-click champion/challenger tests won't work since these won't be triggered campaigns. If you have single emails dragged into your sends, then the standard champion/challenger tests are likely easiest -- although you have to do your own manual reporting on results because once you declare a champion the pretty chart showing results goes away.
Best of luck with this!