Can I add it to an existing save?

Yes. The flock appears around the crashed ship, or around the spawn point when the ship is gone.

Can I remove it from a save?

Yes. The turkeys are removed with the mod.

Does it need Space Age?

No. It works with or without Space Age. Turkeys live only on Nauvis.

Do the turkeys respawn?

No. A turkey you kill is gone for that save.

Can I turn the gobbling down?

Yes. "Bush turkey voice volume" in the per-player mod settings sets the startle and gobble calls from 0 to 1. Death calls keep their volume.