Rocksweden - I changed my formation with the same players a couple of times to counter the other team's formation..
Not sure but got really random results.. but generally not bad..

Qambu - I totally know what you mean. Actually it is true for real world teams too! I play in an indoor team and we were winning many games, dominantly, like 4 in a roll. Then on the next match we did not have enough players and got a fill in player.. Up until that game, we were only getting scored 2 or 3 goals.. then on that match, 4 same players and only 1 guy was different than previous matches. We lost pretty badly like 7-14. So changing one or two players in a squad of 11 can make the team lose balance for sure!