1 pipe rad installed at the top of a 2 pipe system??

Hi, I'm new to steam heat, and I have a house that was built in 1920, 3 story brick on brick construction. The whole house has a 2 pipe steam sys. connecting in the basement to a new (1-3yr old) Utica boiler. My first question, of what I'm sure will be many...(waiting till my copy of Lost art of steam gets here before I ask them all!) is I have a single rad in the 3rd floor that is plumbed as a 1 pipe system. It only has the one oversized (in comparison to the rest of the rad supply pipes) pipe, and a rad air vent on the opposite side about half way up is this ok? I've yet to find any info on a combined system, and I don't know if it hinders the 2 pipe system that is in the 1st and 2nd floors.