The Support Vehicle rules are my standard go-to, unless I'm building a distinctly combat oriented trailer. Outside of the "towed mini-fort" idea (which is fun, but has flaws), nearly all my trailers are not combat oriented. Cargo trailers are simple enough, but also medical or technical support trailers. Especially for combat support vehicles, like artillery, the idea of towing something with an extra few tons of ammunition, is pretty useful.
Designing a tracked trailer is no different from designing a wheeled one. Just use a different motive type, either in the Combat Vehicle rules or the Support Vehicle rules. There's nothing special to a tracked trailer, except for the terrain it can traverse. Personally, I tend to stick with off-road wheeled designs, just because I think it matters less for strategic mobility. Obviously, if you're going to use them for actual game play, when terrain will matter more, and then use a tracked design.
There are no weight limits for trailers, except those already in place for vehicles anyway. Just multiply the tractor's mass by the cruise speed, then divide by the total of the tractor and trailer together (rounding down). So, a 50 ton tractor that normally goes 4/6, pulling a 15 ton trailer will go 3/5 (50x4=200, 200/65=3).
Conversely, of course, you could divide the product of mass and speed by the desired combined speed, then subtract the tractor mass, to determine the viable trailer mass. Let's say a 75t tank going 4/6, that you don't want to slow more than 3/5. So, 4x75=300, 300/3=100, 100-75 = 25t max for the trailer.
