I modded the ship files on a couple different ships
module_slots = {
fixed_ship_battery_slot = inherit
fixed_ship_anti_air_slot = inherit
fixed_ship_fire_control_system_slot = inherit
fixed_ship_radar_slot = inherit
fixed_ship_sonar_slot = inherit
fixed_ship_engine_slot = inherit
fixed_ship_secondaries_slot = inherit
fixed_ship_armor_slot = inherit
#fixed_engine_type_slot = {
#required = yes
#allowed_module_categories = { ship_engine_type }
#}
The # sign negates the line item. this cleared it up