There's a grooming product for horses called Cowboy Magic. It is absolutely amazing for unruly pants. I used to use it for manes and tails, but it's wonderful for Sheltie pants. It goes on as an oil, but is quickly absorbed and leaves the hair totally manageable and shiny. You put just a tiny bit in the palm of your hand, rub your hands together and then run them through the pants. Combing is a piece of cake. It's so good they advertise it now for dogs too. You might not need to thin with this.
Here it is:
http://www.cowboymagic.com/products-detangler-shine.html
If you do trim, I would definitely do it yourself with a pair of thinning shears and just do a very little at a time to see how it looks. I would never let a groomer near a Sheltie, unless it's a Sheltie breeder.
