What model SCUT, and with what attachments?
An 18 - 20 hp SCUT with just a mower deck is going to weigh somewhere under 1000 lbs. The larger the tractor hp and frame, the more it will weigh. Add a loader and you'll add anywhere from 500 to 1200 lbs. depending on the loader model. Add a backhoe and that's another 1200 to 2000 lbs.
You want a trailer that's long enough to be able to hold the tractor and implements without anything hanging out the front, rear or sides, plus you want it to be rated so that the most weight you'll ever have on it isn't any more than 60 to 75% of the rated carrying capacity of the trailer. Anything over 2000 lbs. total combined weight of trailer and load you should have brakes on the tractor. Any Gross weight above 3500 lbs. or so you want a dual axle trailer.
As to your question on the weight being too much for a septic system, it depends on the tractor model, and what kind of tires are on it as to how the weight will be distributed, plus the exact specifics on the septic system need to be taken into account. If it is a sand mound with shallow leach pipes then it will be more susceptible to damage by weight from above than if it is solid ground and/or gravel bed above the pipes, etc. It really is an "It depends" kind of question.
