I agree with
hairyyahoo regarding the differences between the Demolisher and the standard Leman Russ. The Demolisher "fits" in better with a mobile force that's built to take ground. The Demolisher is a close support tank. The Leman Russ is a long range, infantry killer. I arm mine with a Battle Canon and 3 heavy bolters. I place mine in a spot where it has good LOS and use it to deny enemy movement.
As far as do's and don'ts o IG, the best advice I can give is to:
1. Don't spend too much on doctrines. IG aren't Marines. It's better to keep them cheap.
2. Don't neglect your Troop choices for Elite, Fast Attack and Heavy choices. Your basic platoons are the backbone of your army. While it's cool to "max out" on other categories, your Troop choices will often win you games. Period.
3. Don't rely too much on close combat. We are the IG. We rely on shooting, some counter assault and ......more shooting.
4. Do have fun. If your Chimera gets destroyed, just laugh it off. Too many players take this game (I did say game) too seriously.
Just some random thoughts...
~fearlessgod~