There is a little bug you need to be aware of (among many...).

If you are going to add trade ships to a fleet on a node, remove the fleet from the node, add the ship, then move the fleet back to the node.

Also, never, ever, remove a ship/ group of ships from a fleet on a node. The remaining ships on the trade node will go kaput. They will show on the screen, but they don't generate income, you can't move them, you can't do anything but click on them... Another one of those bugs...