I’m far from an expert on cold hardy pears but for starters we know the pears you mentioned are hybrids of Pyrus ussuriensis x Pyrus communis. Some nurseries specialize in cold hardy pears such as http://www.sln.potsdam.ny.us/pears.html. SLN nursery uses two rootstocks which they state “The rootstocks we use for our pear varieties are Pyrus communis (for European-type pears) and Pyrus ussuriensis (for ussuriensis crosses)” . Your zone is very cold at times in the winter so I suspect it would be wise to stick with harbin pears as rootstock on the hybrids your planning to plant due to their cold tolerance and because the scions are a hybrid. Pear decline can be an issue with the harbin pear aka Pyrus ussuriensis. You will already be growing a variety that’s a cross of Pyrus ussuriensis. You can get those pear trees here http://www.sln.potsdam.ny.us/pears.html. You likely already know “OHxF 333 (Old Home and Farmingdale) Produces a tree 75% of standard size. Resistant to fire blight and pear decline. Does not sucker and develops hardy well anchored tree. Tolerates a broad range of soils. Bears fruit in 3-4 years and grows to 12-16 feet.” - per http://www.treesofantiquity.com/index.php?main_page=page&id=6 . Old home is half of that cross and farmindale the other half http://www.ars-grin.gov/cor/cool/pyr.oldhome.html . There are several types of OHxF crosses as shown here http://copenhavenfarms.com/cherry-plum-and-pear-rootstock which are reasonably priced rootstocks http://copenhavenfarms.com/cherry-plum-and-pear-rootstock-price-list. Rootstock heights etc can be seen on the pear link on this page http://treefruit.wsu.edu/varieties-breeding/rootstocks/. You also need to know cold hardiness which I could not find a definite answer to how cold an ohxf rootstock can get before it dies http://whatcom.wsu.edu/ag/documents/treefruit/pnw341e.pdf. I would stick with the tried and true methods in your area for now. I will say most pears are compatible with rootstocks or compatible with an interstem that’s compatible with a rootstock so other options may be available. Hope that helps!