ok I have found a fix so you can place your own turrets in covenant. You use the console command disable to remove the bodies and the remains of their turrets. Then you're going to want to make a tall ladder. Put the ladder against the outside of the wall next to where you want to put a turrent. Now when I tried to do this it still showed it I certainly did it in my first playthrough. The build limit at every settlement is specifically there to prevent too much going on at each settlement, thus averting performance problems. So if you're suffering from crashes specifically as a result of building up settlements (and this is a big unknown, given that FO4 can crash due to a myriad giCXO.
