Start by modifying the newbie slot so that it is actually used by a newbie, and not as a 26th slot for an experienced player by some.
After 3-6months, a newbie is not a newbie anymore. And I have never seen a corp that didn't have 1 player stopping in any 6month-period. Fix that, and we could maybe discuss having more newbie slots.
There will be some benefit from this.
But this will not help balance the game.
An experienced player can always register a new account and play as a newbie.
Not the months determine a player is a newbie. The stars are a better indicator.
But according to the rules, the place of a newbie in the game is reserved! Regardless of his achievements in the game!