No, farmsteads should not shoot! (see what I did there? should/shoot, sorry xD) I would even say only civilians/civ-soldiers should be able to garrison inside. Not allowing CC to be build on farmland seems a bit strange. I would rather place a tooltip warning like: 'WARNING: Placing a civ center on this farmland will degrade the soil!' or something. And when you do decide to place a CC there you just lose the farmland bonus.