Think of a large pot of water. Will a circulator anywhere under water have difficulty pumping water to the top of the pot? Your heating system is filled to the top, so it doesn’t require a circulator to pump water to the top. A circulator only has to overcome the resistance of the length of the pipes and the turns in the fittings, so your circulator does not have a problem pumping at different levels. It does have harder work pumping through small tubes such as the fin-tube baseboard run and the resistance of the kickspace heater because the smaller holes in the tube create a resistance to the flow of water.
Although it doesn’t matter as much in a basement cast-iron radiator, because the basement is a large area less sensitive to temperature swings, it is not a good practice to put cast-iron radiators on a lightweight fin-tube baseboard loop because the cast-iron retains heat for a long time after fin-tube loses its heat to the room. A thermostat in a cast-iron-radiated room will sense that the room is dropping in temperature long after copper-tube baseboard in another room on the same loop has given up its heat – the other room will be cold.
If the bedrooms are set back in temperature, the basement radiator will be giving off less heat at the same time if it is on the same loop. If this is not a problem, then the cast-iron radiator on the loop with the other cast-iron radiators is a good place to join the basement heat. If the basement needs heat at these times then a new loop with its own circulator is necessary.
You can learn more about hot water heat in the free lessons at http://www.hydronic.net