Wellington, West Rider has given you some excellent feedback. Here is my response to your question about what makes up a well balanced guard force. The IG are, as people have noted, a very diverse force. I like to organize my lists around a given theme:
1. Light Recon: Command squads, platoons, conscripts, Salamanders, and scout sentinels often work well with this theme. This is a light force designed to locate the enemy and break contact (so only light tanks). I also like to to use the Emperor's Talon Recon formation from Page 40 of the Montka book (this formation was discussed previously in this thread). At 1850 points I often include 130 to 150 troopers, a couple of Salamanders, and 12 to 15 scout sentinels.
2. Recon in force/Punitive Raid: Command squads, platoons, conscripts, Salamanders, and scout sentinels as before but I also include heavier units like Russes and Mechvets. This list is designed as a heavy recon force designed to move swiftly to find and destroy the foe. I avoid arty or ordnance tanks since this is still designed to be a recon force (and Imperial Armor Colume I noted that recon units sometimes tale Exterminators along). At 1850 points it includes maybe 5-6 tanks, 100-ish troopers, and some sentinels.
3. "Bridge too Far" List: Command squads, platoons, and conscripts, supported by stormtroopers brought as troops using the MT Codex. This theme is inspired by the British Red Devils 1sr Airborne Division at Arnhem in WWII, so no tanks or walkers. It features tons of light infantry and drop troops holding out against overwhelming odds. At 1850 points I have maybe 130 to 140 line troopers and 30 to 40 deepstriking stormtroopers. This detachment also fares about as well in games as the Brits' para and glider troops did in real life during Market Garden. :)
4. Blitzkrieg Force: Command squads, platoons, conscripts, mechvets, Russes, self propelled artillery, and armored sents. This is a classic treadhead list. In 1850 points I have usually 10 or 11 tanks and 80 to 100 troopers. For flavor, at least one of my platoons is usually meched up and mounted in Chimeras.
5. Urban Combat Detachment: Command squads, platoons, conscripts, veterans, and "urban " themed tanks (Demolishers, Eradicators, Hellhoonds, and mortar tanks like Griffons or Wyverns). I don't mount up any troopers since fast mounted units don't seem to fit an urban theme. I always include plenty of flamers and sniper rifles and a ratling squad is a must. At 1850 points this list usually has about 150 troopers, 4 or 5 tanks, and some scout sentinels.
6. "Classic" Force: A mixed force of command squads, platoons, conscripts, and mechvets with various and sundry thrown in. In 1850 points it features fewer tanks than my Blitzkrieg lists, maybe 5 or 6 tanks and about 100 to 150 troopers. It represents a force that can easily adapt to holding the line or attacking.
Almost all of my lists feature at least 1 infantry platoon, 1 conscript platoon, and 1 mechvet squad or deepstriking MT scions squad. I find this variety works best for me as each specializes in a respective task.
-Platoons: ranged shooting buffed by orders, bubble wrapping, and holding homefield objectives
-Conscripts: tarpitting, midfield objective grabbing, gaining board control, and bubble wrapping
-mechvets and scions: killing enemy elite units and armor with melta and plasma, grabbing objectives in midfield or in the opponents' deployment zone
I hope this helps.
