Hi Jones!

Using the same players each game has the advantage, that those player getting better rating, experience and teamchemistry. It doesn't effect the stamina and the energy. The energy only goes down while training, stamina goes up when trained.
The disadvantage is, that your other guys don't improve rat, exp and tc, which effects how good they are playing.
It's needed to have some backup players in the rooster, because your players could get injured. So you have to have guys to play for them. But always take a look at your economy! Too many players leads to too high wages, could lead to financial problems.
Use your experienced players for league games and play your friendly games with the new guys to get them better.