It's funny that this topic was made, because I was just recently fiddling around with the idea that YMX is Isa in some form.
I took pictures of their models and overlaid their eyes/faces on top of each other to see how similar they were, in the exact same manner that I did when I put Vanitas' eye/bangs over Sora's face to show that he probably looked like KH2 Sora.
Unfortunately, I didn't save the picture because I was disappointed by the result. From a cursory glance, YMX definitely looks like Isa, but when you compare their faces, they aren't the same. Isa has more rounded eyes and thicker eyelashes while YMX's are more thin. Additionally, their chins to line up exactly and Isa has a thicker lower lip. But I can see why someone would think they look similar (I did, and that's why I compared them), because they both have that sort of half-lidded stare and YMX's manner of talk is a lot like Saix's.
So I would be quick to call this an open and shut case, that YMX is not Isa/Saix.
But.
There's one thing I think people forgot about, and I mentioned it months ago.
Not sure how reliable this was, but according to a hacker, when they were extracting the texture kit for MF's model from BbS, they found it to be the exact same as Saix's kit from KH2. The thing about Saix's texture kit is that, while many of the Organization members have the same textures just at different proportions, Saix's kit is one of the few that is unique because one of the things it has is his earrings. And that's what they found on MF's texture kit. Apparently, they used Saix as a reference.
I still have pictures of the kit on my laptop, actually, might upload them later.
That little tidbit, assuming it is accurate, will probably throw this topic into chaos, but I thought it best to inform you guys!