There are likely to be very significant swings in temperature in a home with cast iron mixed in with steel radiators. Here’s an example: when you have the long run-cycle of cast iron radiators, and there’s a steel radiator in one room or area, that steel rad heats up with the rest of the radiators. But once the thermostat is satisfied the cast iron rads are sitting there nice and warm for 45 minutes, the steel rad cools off in about 10 minutes, and the room temperature starts to drop. If the thermostat is in the room where the cast iron rads are, the thermostat will not tell the boiler to come back on for another 35 minutes.
There are ways to rectify the problem. If a client is building a new house, and they outfit the house completely with steel rads, the entire operating style would be different. The boiler would cycle more often and does so for shorter periods of time. It will come on for five minutes, heat the rads up, stay off for ten, come back on, and you end up with multiple shorter cycles over a period of time.
If you wanted to put an addition on your home and use steel radiators in the addition but have cast iron in the existing part of the house, you would create two separate zones for the house. The addition would have its own thermostat controlling the steel radiators and the cast iron radiators would also have their own thermostat. The two zones can operate independently of one another, using one boiler.
The typical set up would that the piping arrangement in the boiler room is designed to ensure adequate flow through the boiler when either zone is calling for heat. A zone control would be present to distinguish which part of the house is requiring heat and directs it to the appropriate radiators.