steam system with one pipe and two pipe?

It isn't really that unusual...

to have a combination of one pipe and two pipe.  The one pipe parts one handles just as one would any other one pipe -- good venting on the mains, good pitch on everything, and control the radiators with their own vents.  The two pipe you handle just as any other two pipe -- there being, however, a variety of flavours of two pipe systems.  Make sure your radiator traps are good; if there are no crossover traps from mains to dry returns, the mains need venting, but otherwise the don't -- dry returns always need venting somewhere (usually at the boiler), and if any returns go direct to a wet return, they need to be vented at a level well above the boiler -- basement ceiling is typical.



Otherwise, so far as installing a new boiler would be concerned, it's the usual suspects -- make sure the near boiler piping is correct, and make sure that the new waterline is very close to the old one (which may require raising the new boiler on blocks or something).  Hook up the old steam mains to your new header (drop header if you can!), the returns to the boiler's return, and fire it up.



Oh yes -- and keep the pressure low!