In my unfinished greenhouse, I planted in my 8' x 3' x 30" raised garden box a Meyer, Okitsu Wase satsuma, Owari satsuma, Kishu mandarin, Page mandarin, Ponderosa lemon, and a couple others that all died when winter temps hit -15C for over 2 days at least twice and stayed below freezing for almost 3 months. Most of these were my oldest, biggest, healthiest, most productive (i.e. favourite) citrus in my collection so losing them was a terrible blow. I've since added another 5 or so trees to my collection to bring it back up to 32 trees, but mainly going to let these grow out cause some are survivors that lost 80% of the plant during the winter and are just recovering now.
mksmith, based on my experience, I would add strings of old incandescent Christmas lights around each citrus tree in your hoop house and plug those in for added heat when temperatures drop in the winter.