I love my tug. Perfectly geared to pull and slow the hack without sidecar brakes. To answer your question on a training sidecar, I've never heard of one, but I think the bike is the more important factor in making the entire rig more of a "trainer" situation. The gs 850 g is a strong tug so I have to be conscious of speed on all right turns. Otherwise, just put more weight in the sidecar to keep it down and I'd say you probably want a little more than a 550 if you want the ability to go more than 60mph. The gs850g will do about 95 or 100 with passenger but with low miles, new carbs and larger jets.
my videos on youtube with american idol winner -taylor hicks https://www.youtube.com/watch?v=-cF2oDtMQBc
The SoberSidecar 1980 gs850g w/ ural or bmw hack