The easiest way is to not have super-soliders at start at all.
All stats are in a specific range.
As time passes, and game moves in more advanced phases, slightly better soldiers become avilable. But their stats are allways smaller than what a soldier kept alive from the start would have.
Fpr better example, at start you get 50 lvl1 soldiers.
3 months in the game, some of your starting soldiers have reacheld lvl8
You start getting recruits wiht lvl5 stats
5 months into the game you best soldier is lvl10
You start getting recruits with are lvl7.