I'm a huge rivet counter and tank fanatic. My quest in this hobby has always been to get a scale accurate 1/30 tank collection. At one time or another, I've owned most every tank by every manufacturer.
I used to post threads here and at Hobby Bunker where I did the 1/30 scale math conversions of different tanks, measured the models, and then posted my results. Search for them if you want to review all those.
The short answer to your question above is:
- Most all of the First Legion tanks are an accurate true 1/30 scale (Opel Blitz, Sd.Kfz. 222, Sd.Kfz. 232, Sd.Kfz. 7 Prime Mover, Panzer IV F1 & F2, Panther, Tiger, Sherman, Jeep, T-34/76, T-34/85, T-70).
- The First Legion Stug IIIs and Panzer IIIs are slightly oversize (somewhere like 1/29 or 1/29.5 scale) as is the Panzer IV Ausf H and Stug IV.
- All the John Jenkins tanks so far are an accurate true 1/30 scale.
- The War Park Panzer III, Panzer IV, Tiger, Hummel are all slightly oversize (somewhere like 1/29 or 1/29.25 scale depending on tank)
- The Xuan Hua tanks have all been accurate true 1/30 scale so far
Remember, the difference between a 1/29 scale tank and a 1/30 scale tank is a couple millimeters. Nowadays, the scale discrepancy is so small, it's not something I even worry about anymore, unless I am in a mood to pick nits.
In my opinion, the scale differences today are so small it is a non-issue. I mix and match JJD, FL, War Park, Xuan Hua, and W. Britains tanks all the time with their figures. I also use TGM tanks/vehicles.
If a combo does not work, like the "large" first release War Park Grey German tank riders with a JJD Stug, I just don't worry about it in the least. I would just use FL grey German tank riders and the "small" later release War Park Grey German tank riders. There's been so many figures and vehicles released at this point, that it's simple to find size compatible combos when mixing and matching manufacturers. Just accept that not EVERY combo will work, and don't try to force a round peg into a square hole.