It turns out that the so-called "6-man tag" glitch was simply a manifestation of the all-too-real failure-to-save bug. It just happens to (usually) take hold before the match even begins (hangs on the "now loading" screen).
Also, I may be making (yet another) Mewtwo CAW, but with a weight-class-optimized chain-grapple moveset (eliminating weight-class-detecting moves), for maximum impact. However, I believe only a single move needs to be replaced, and that is the Triple German Suplex, because of the severely jerky motion when performed on characters with a lower weight class.
The Mewtwo I will use will be based on a fusion between Mega Mewtwo X and Y (X's size and Y's appearance), so he will be a super-heavyweight, with truly maxed-out attributes (OVER 100, in that every attribute except strikes will be maxed out at 100).
To achieve the true nearly all-100 attributes (which does NOT freeze the game--I tried it a couple of years ago), create a CAW and give him the light-heavyweight weight class (anywhere between 200 and 239 pounds). Slide all attribute sliders as far right as they will go, then go back and set his weight class to super-heavyweight (280 pounds and up). Then, slide any other attribute sliders not at maximum to their limits. You'll have all attributes at 100 except for strikes, which will still be at 95.
This will give your character a true overall of 109 (the game will still show "only" 100). He will be easily capable of destroying even the likes of Undertaker, The Rock, and Stone Cold Steve Austin (the three highest-overall non-CAW characters in the game).