There is no doubt that the tractor will start if you hold the glow plugs on long enough, however, diesel engines don't like cold oil, and cold engines suffer from more wear than a warm engine. I have owned diesel engine cars since 1966, and I have seen how much both the fuel and oils have changed, however, the one thing that hasn't changed, is what they sound like when they are started cold.
If you go with a magnetic heater, you can place it anywhere on the engine that is convenient, as long as it makes good contact with the surface. Since I have a freeze plug block heater, I have never had a need for a magnetic block heater for my Kubota BX22. I made the assumption that the engine oil pan it steel, and that is where I would place the magnetic block heater, but if it is aluminum, then you will need to find another location. I know that they used to sell a heater that you inserted down the dip stick tube, but don't know if those are still available. In the early 1960's, when I was in school, and the temperatures got into the negative numbers, we would have to put a shovel of hot coal embers under the oil pan to warm the oil enough to start the car. You always wanted to make sure that there were no fuel leaks, and that the engine wasn't a greasy mess.
I also use a full synthetic oil in my Kubota, which greatly aids in reducing start up friction.
You have to do what you feel comfortable with since ultimately, you are responsible for your own tractor and its maintenance. If you feel that glowing the glow plugs for some extra time is ok for your cold engine, that is fine with me. I don't expect everyone to agree on the subject, I am just relaying my experience and views.
https://www.youtube.com/watch?v=1gmuHXQvXI8