I've been sitting on my Pulse save game for a while doing the monster quests, and want to know if I can advance the storyline and come back to fight more monsters later.
Yes, you'll have a chance to return to Pulse to finish sidequests, explore, and generally goof off just before you fight the last boss. Beating the game will also unlock the final level of your Crystarium, at which point you can grind to your heart's content to max out your party.
I believe this is possible after you finish the main story line.
Yes you can go back. There is a Portal in Orphans Cradle, which is the area before the final boss (Chap 13). There are two portals, on is to Eden, the other to Gran Pulse