2 Allow us to specify a unit as replaceable but not recruitable at a building.
One way to do this (at least from a human player's perspective) is to give the unit a build time of 999 turns (or some rediculously huge number). Retraining still happens in 1 turn, but you won't have time to even recruit 1 of the units during a game. The only thing I'm not sure about with this method is how the AI would manage with it.