I would not feel comfortable using most scissors around a cat. They can be unpredictable and jerk at the most unexpected moments (imagine a quick head jerk back to bite at the scissor and end up w/ a poked out eye), and I don't think you'd get a great result.
I don't believe the guard coat is really what makes them very warm and traps oil close to the skin, but rather the much finer undercoat. Something like a furminator would help w/ shedding and lighten that up considerably.
If you have serious matting issues or butt-danglers, then you can bring kitty to a groomer, but its not a true long term solution. Cats need to be able to lick their tail end to avoid hotspots and especially genitalia as their saliva has enzymes that break down bacteria and prevent infection, be it longhair or short.
So the real answer would be a weight loss program, but no more than 1-2% of body weight per week, else your kitty can suffer liver damage. Nearly half of all cats in the US are overweight though, so there is plenty of support and ideas out there. The best is to go onto scheduled feeding for calorie control, and preferably start feeding more wet food and a low-carb kibble (grainfree variety typically are) is also helpful.
