Fin tube baseboards need not be used. Cast iron baseboard and radiators are still available and could be installed.

You certainly can use copper-fin tube baseboard on a cast iron radiator system, but it is pretty complex figuring to get the right levels of heat because its properties are so different.

Cast iron baseboard is great stuff, but the last time I looked a couple of years ago it was $70 a linear foot.

Thank you for the info, I'll research the cast iron. I'll still have the option of the little electric space heater, they make me a little nervous because someone always forgets to shut them off.

You have another option, too, one that might be better overall...

It would require some repiping, though.

Break the system down into zones.

In areas where there aren't any heaters currently, consider using toekick style heaters, which use an electric fan to blow water across coils heated by your boiler.

The expense and disruption would likely be much less than installing cast iron baseboard, and you might be able to do the runs with PEX instead of copper or iron pipe, which would be a huge savings in labor.

Because castiron is less conductive than copper/aluminum, castiron is a more "Gradual" type heat taking longer to heatup and longer to cool down. If you combined fintube with castiron, those rooms with fintube would heat up quicker and cool down quicker. If you left the thermostat in a room with castiron heaters, the fintube rooms would have a much greater temperature fluctuation resulting in more hotter & more cooler than the rooms with castiron. Because castiron holds the hot water after the boiler/pump stop (System Satisfied), the castiron continues to radiate heat until it is cool or the system calls for more heat. If you move the thermostat to a room with fintube, the boiler/pump would cycle more often, causing the castiron rooms to overheat.

My first home was built in 1888, a 1 3/4 story house, and the previous owners had removed all the castiron radiators and installed a single zone fintube system with a Wards boiler. The Kitchen & Dining was the last on the loop and was always cold, the upstairs was first on the loop and over heated. So I replaced the boiler, provided 3 loops (Upstairs, downstairs & Kitchen) and off each loop, every room had a separate zone (Honeywell Thermostatic Valves) and balance valve. The Kitchen had a toe kick heater. Every room was very comfortable and I could turn heat down in spare bedrooms.

You can find castiron radiators in a Salvage yard or reclaim center.

The link has ratings/limitations on various PEX tubing.

Here is a link that might be useful: PEX Ratings