Howdy kaomera!
S'mon has already answered most of the questions.
As for Rank Scaling, the key is to use the most appropriate Rank closest to your PC's level. When the PCs are directly in the middle between what makes the best rank, then always change the monsters rank to the higher level.
Solo Monster = Standard Monster -9 Levels
Elite Monster = Standard Monster -4 Levels
Minion Monster = Standard Monster +8 Levels
Example: Young White Dragon
= Level 3 Solo Monster...use when the PCs are between Levels 1-5
= Level 8 Elite Monster...use when the PCs are between Levels 6-9
= Level 12 Standard Monster...use when the PCs are between Levels 10-15
= Level 20 Minion Monster...use when the PCs are between Levels 16-21
Now you might be worried about using a lot of minions (as per S'mons example) might take too long to run. The solution here is to use the UNIT rules, whereby you treat a group of monsters in the same way as a swarm.
So instead of running 50 gnoll minions, you might run 5 Mobs of x10 Gnolls. Wherein each mob is a single Standard Monster rather than treated as individuals.
You can scale this idea up so that instead of fighting mobs of x10, you could fight companies of x100, battalions of x1000, legions of x10,000, armies of x100,000 or horde's of x1,000,000.
Of course using this method you can easily pit opposing armies against one another.
The question becomes what level are all these different units converted. Here I think you have to break from the XP mechanic used in Rank Scaling and start taking diminsihing returns into account. As best I can pinpoint it, +6 levels (for each factor of 10) seems to be the best fit.
So if a typical Orc is Level 3.
- A squad of 10 Orcs = Level 9
- A company of 100 Orcs = Level 15
- A battalion of 1000 Orcs = Level 21
- A legion of 10,000 Orcs = Level 27
- An army of 100,000 Orcs = Level 33
- A horde of 1,000,000 Orcs = Level 39
Of course then you have to determine how powerful the groups leader is. It doesn't really make sense that the leader would be higher level than his most powerful unit, so I suggest the leader should be about 1-3 levels lower (of course they could still be Elite or Solo monsters).
Here are the sample Orc Leaders:
Commands less than 100 Orcs = Max. Level 8...Orc Chieftain
Commands < 1000 Orcs = Max. Level 14...Orc War-Chief
Commands < 10,000 Orcs = Max. Level 20...Orc General
Commands < 100,000 Orcs = Max. Level 26...Orc King
Commands < 1,000,000 Orcs = Max. Level 32...Orc Emperor
...of course the leader doesn't have to be the same race as his subjects. Instead of an Orc Emperor you could have an Ancient Red Dragon.