The true plot of FF8!

[Masamune·1600]

Here's my original suggested master-pet pairings:

Squall -> Griever
Rinoa -> Shiva
Zell -> Ifrit
Selphie -> Carbuncle
Quistis -> Siren
Irvine -> Alexander

Seifer -> Diablos
Raijin -> Quetzecoatl
Fujin -> Pandemonia

Some people took exception to the Irvine pairing, but there's no GF I could find a closer resemblance. Others thought a less evil-looking GF would be more suitable for Seifer.

Of course what better GF for Selphie than a bouncy green bunny?

I know you've already stated that the character -> GF theory is unlikely, but there are several things I would like to point out as regards this idea. First, the most common suggestion is that Squall is Griever (usually in conjunction with certain R=U theories). This, however, is in and of itself false, as we know that Ultimecia constructed Griever from Squall's thoughts.

[...]the game directly addresses the question of Griever being directly Drawn from Squall's mind. Ultimecia tells Squall that Griever is being Drawn from his thoughts. The direct Japanese dialogue drives this home with heightened clarity.

Quote:
Ultimecia: Your feelings, I shall summon the most powerful of things
[from them]! The more strongly you feel, that will be what shall torment
you. Fufu.

In this context, the scan data can be interpreted to support the supposition that Griever was "created on the spot." Griever, in Squall's mind, was the ultimate GF, and, using those thoughts as a template, Ultimecia brought forth a GF to battle the party. This view makes even more sense in light of the fact that Ultimecia can blow away stocked spells, suggestive of her ability to affect memory.

It's also important to note what Griever actually meant to Squall. On Disc 2, when Rinoa questions Squall about his ring, he notes that lions (like the one on the ring) are known for their great strength and pride. As these are qualities that are very important to Squall, I would personally view Griever as more akin to an ideal, than simply a known legend.

Quote:
Rinoa: You know something, I couldn't afford to fall off that cliff and
die. I have something very important that belongs to you. I can't die
until I give it back to you, right? Zell gave it to me, see? I've been
holding onto it.

Squall: (I'm gonna kill him...) That's my favorite ring. You'd better
give it back.

Rinoa: I'm sure it is. It's a cool-looking ring. What's this monster on
it anyway?

Squall: It's not a monster. It's a lion. Lions are known for their
great strength and pride.

Rinoa: Hmm...great strength...pride... ...Kinda like you, Squall.

Squall: I wish...

Rinoa: Hmm...so this L I O N of yours, does it have a name?

Squall: Of course. Griever.

Admittedly, we know that Squall has some knowledge of 'legendary Guardian Forces.' After all, when encountering Bahamut in the Deep Sea Research Center....

Quote:
Bahamut: I am... Bahamut.
Squall: The legendary GF....

This is not enough, however, to sway me from the belief that Griever was, in fact, directly Drawn from Squall's mind. It's certainly possible (maybe even probable) that a GF called Griever existed in the world of FFVIII. However, the Griever utilized by Ultimecia was, most likely, a reproduction of this being based on the parameters for Griever that existed within Squall.

This bit may seem out of context (and it is, as it was originally a reply to the idea that Ultimecia's Griever was not formed on the spot/Drawn from Squall's mind). In this case, though, the idea is clearly to demonstrate that Griever is not Squall. Having said that, the segment concerning Bahamut as a "legendary GF" suggests that at least some Guardian Forces are known, and can be demonstrated to be static. As I see it, at least, this theory can't work.

There's no explanation of who Tiamat is junctioned to in the present, but it would be natural that there are other GF's out there that the party doesn't get.

It's almost certain that GF's exist that the party never encounters. Selphie herself makes mention of junctioning a GF before coming to Balamb (this was included to explain her memory loss, although we can use it here to suggest the existence of other Guardian Forces).

I've always liked the notion that the uncorrupted Tiamat could have been Selphie's mysterious GF; unfortunately, there's no way to know.

I've considered the link between Ultima Weapon and Omega weapon, but what I don't get if they're the same monster then where did Cloud's sword go?

They're not the same monster. Scanning Omega Weapon suggests that he is more powerful than Ultima Weapon, which in itself suggests differentiation between the two.

Stronger than Ultima Weapon, Omega Weapon is the strongest monster. It is bad luck to run into this monster.

For that matter, Ultima Weapon can be destroyed before Time Compression is initiated, which would mean that, if they were the same, Omega Weapon would have already been destroyed. While the normal flow of time is distorted during TC, this event (defined before the distortion), would have already been locked in stone.