I have the Shen Hao HZX45-IIA as well, my favourite camera, I also have a Toyo 45G monorail. The Toyo has a bit over 400mm in bellows so I can get quite close for portrait with a telephoto lens.
I have a Komura f/6.3 400 telephoto lens, it weighs 898g Within reason it works well on the Shen Hao, but it is a big bit of metal and glass hanging off an extended front lens. It's best point is to be able to draw subjects closer to you out in the field if using the Shen Hao. For portraiture it is clumsy if you are trying to close focus, when in the Shen Hao. If it is on the Toyo, it works quite well for portraiture.
I also have a Schneider Kreuznach Linhof Tele-Arton f/5.6 250mm lens, I picked this lens up to replace the Komura as it was much smaller and therefore better for the Shen Hao. Well it was better being smaller and lighter, but it was limiting with movements; frustrating would probably be a better word. Great for close up portraiture, but not brilliant.
Eventually I picked up a Fujinon f/6.3 250mm lens, this is outstanding, really outstanding. It has a wide coverage so anything I can do on the Shen Hao or Toyo as far as movements go, it handles with ease. It is pin sharp and it is short enough to allow me to extend the Shen Hao bellows for close portrait work. Handles landscape, architecture and portrait work admirably. Effectively, I have retired my Fujinon f/5.6 210mm lens as it is just that little bit too short.
I just pulled out the three lenses and weighed them on the kitchen scales.
Fujinon f/6.3 250mm 653g
Schneider Kreuznach Tele-Arton f/5.6 250mm 692g
Komura f/6.3 400mm telephoto 898g
I looked at getting a compact 300mm lens, but I couldn't find any that were in my price range. I tried a compact 300mm lens from a friend, it is probably the best all round, but the price and availability for me didn't happen.
Mick.