I have a TC29D which weighs around 2500lbs and a Ford model 515 sickle mower with a 7' cutter bar which weighs around 500lbs, so the tractor weights about 5 times as much as the mower (I’m rounding everything to the nearest 100lbs). It handles it easily, but wouldn't want to go any heavier. It can get a little squirrelly on steep hills.
According to my NH brochure on Boomers, the TC18 weighs around 1400lbs, so if you used it with a 400lb mower the tractor would only weigh about 3.5 times as much as the mower. It's true that part of the mower rides on a skid plate on the ground, but there are times you need to pick up the bar to get over obstacles and transport it. I think it would be a good idea to get a mower well under 400lbs for a TC18, unless you add a few hundred lbs of weight to the tractor.