For the two-member party, you should try starting with a commando and ravager, and then switching to ravager-ravager once your commander has landed a combo, until you stagger the enemy, after which you can switch back to COM-RAV. This is the fastest way to kill enemies. It should work on most standard enemies, anyway. It's been a while since I played the game, but I definitely remember using the COM-RAV-RAV for my three-member party and then immediately switching to RAV-RAV-RAV until the enemy is staggered. This works until you get to the Archylte Steppe, where you'll need one sentinel pretty much at all times if there is more than one enemy.
I picked up basically what you described yesterday. I had run into problems with RAV-RAV parties letting the chain expire, and I didn't realize that the Commando chain retention effect would stick even if they changed roles.
I've been starting with SYN-SAB (if I've got them in my party, and if the enemies I'm facing are a threat), moving to COM-RAV, switching back and forth between RAV-RAV every second cycle, and moving to COM-MED or RAV-MED for healing (depending on how the chain is doing). I'll see how SEN fits in once Snow is back, but they're basically tanks, right?
Yeah, Sentinels are tanks. You'll be needing one once you get to the Steppe. Just play as you do normally once you get there (after saving) and you'll know what I mean. Try to get as far as you can without using Sentinels. There is quite the difficulty spike at that point; you'll see why they're essential.