I must've missed something along the way, but what is it with certain enemies' Stagger meters? Sometimes their meters deplete REALLY fast before you can get them Staggered, other times the same enemies' meter will charge very slow. It seems to me that this is more of a issue when I have a party with 2 Ravagers, which seems backwards to me. If I do a Ravager/Commando combo, it slows the meter down, then I will switch to Rav/Rav to fill the Stagger meter, then switch back to Com/Rav to do some damage. What is happening here?

Ravagers get the meter up fast. Commandos keep it from emptying. Read the info about each class. It tells ya more.  
 
Ex 
 
Ravagers can get it up very fast. However with no commando to keep a " Place holder' The bar will empty very fast. 

Ravagers make the meter go up faster, but it also drains faster.  Commando's don't boost the meter much, but slows it down when it drains so you can hold the meter longer.  For most enemies, get in a few commando/ravager attacks until the meter is around 30-40% to make it hold without draining too fast.  Then switch to ravagers for quick boosts to complete the meter.  It's nothing weird that's happening, just part of the strategy.

Use a COM with your RAV or switch back and forth to keep the meter from draining.