seems baseTradeIncomeRate 1.000000 is the only thing that effects income rate.
the Cargo ships are just pretty Graphics
maxCargoCapacity 50.000000 in FrigatePsiTradeShip FrigateTechTradeShip FrigatePhaseTradeShip
is only there to give other players money when they kill your ships.
Tec Cargo size research just increases baseTradeIncomeRate
You need maxCargoShipCount at at least 1 to get any credits but you can set the respawn time to a point where its off.
so to answer your first question.
use these values
at the end of each races tradeport and starbases and you will effectively turn off trade ships