From my basic understanding, you get this grid by beating all of the monsters in an oversouled state and then view all of them in Shrina's bestiary. My question is does this also account for the bosses in the via inifinito in Bevelle on floors 20,40, 60, 80, 100, meaning do I also need to get these monsters in a oversouled state before defeating them, excluding Trema of course? I am guessing the answer is yes because consequently they are in the bestiary after defeating them. Thanks

Yes I think you do, every monster in the bestiary needs to be oversouled and beaten for you to get that Garment Grid I think. I'm not 100% sure because I haven't got it, but I'm pretty sure.

The Piggyback Official Strategy Guide [well, the Italian version, at least] states that you have to get every monster in the Bestiary in an "Oversouled" state, it makes no mention of beating them while under such condition, though. Damn, it's been a while since I've played the sequel.

EDIT:

Okay, read the guide's part dedicated to that 'dungeon' in Bevelle: sorry to say this, but yes, every Boss besides Trema may go into the "Oversouled" state. Ah, don't forget about that giant Cactuar/Kyactus in the Bikanel Desert, either, that one goes into an "Oversouled" state, too.

You must oversoul every single monster in the game, with two exceptions: Georapella and Precepts Guard, which just need to be fought in the Bevelle Underground. The Via Purifica contains all monsters excluding those two. Then talk to Shinra on the Celsius and go to Shinra's Bestiary. Google is your friend.