I don't want to sound rude, but did you make all of these? If you didn't, you need to at least give credit to the original artist.
That aside, I don't think any of these are acceptable. It's not that they're bad, far from it; I really like mega Pidgeot. It's just that none of them fit this resource.
Mega Mewtwo x & Y: While some of the linework is questionable, I like these. With some QC, they may be useable, but that's up to MrDS. The backsprites on the other hand, are unusable. They're good, but they're in gen V style. They may make a good reference, however.
Mega Diancie: Everything is in the right style, but it's far too small and cramped. The backsprite might be usable, but the entire thing is just messy.
Zygarde 100%: Same problem as M-Diancie: It's too small and cramped, leading to the sprite looking sloppy.
Mega Pidgeot: While I really like this sprite, there are a lot of things wrong with it if you look closer. The wings on the backsprite look too small, and the tail is in a weird shape, some of the linework is sloppy, and of course, it's in the completely wrong style.
Sorry if I sounded a little harsh.
Edit: Are we even going to do the Zygarde forms? I assumed we were because they're part of gen VI, but I wanted to make sure.
Edit 2: QC of capylus's mega Tyranitar sprite:
Edit 3: QC of the mega Mewtwo Y sprite uploaded by Versekr Dark: