Mah, let's play all of them together because they are all playable ... oh wait, there is is limit to four Magnezones. Do you want to have a deck with 1/1/1/1? Don't think it works very well. If one is knocked out, you can't have an instant backup. This is the main drawback to every Magnezone and Dusknoir deck.
Without IFDS, Magnezone is a nice card but I really doubt it to be one of the top decks. Damage output from DP Magnezone isn't very good - max 80 for 3 if you manage to get all of your attached energy metal. So you have a stage 2 Lv.X as your only really good hitter - this has never has worked yet.
Kingdra will win because it's faster and has better recovery. I like Magnezone and the possibilities of that deck, but it needs to much setup to have its full potential.
I'm not saying to play them all together. Play a combo of them that fits your decks strategy.
Are you kidding me? You're saying that a three for 30 attack isn't very good?
You could easily play a 2-1-1 IFDS-DP-Lv X line. You've got your Pokemon set up, free retreat, and mass energy manipulation. All that you would be missing is a prime back up attacker.
Speaking of which, who says that the Lv X has to be your only really good hitter? I can easily play some kind of back up attacker with it that can max out at more, or aid set up making the deck even faster than it already is.
Remember, you set yourself up with Magnezone IFDS. It will let you run Metal and Lightning type techs to your hearts content, and makes it incredibly easy to set up your back up attacker.
And towards your point on "A stage 2 Lv X as your only really good hitter has never worked before", well, you should probably go check the Deck Help forum, find the Rhyperior Dp-on thread, and give that a read through. I tested that deck ALL day today at league, losing only once because of decking myself. And that decks ONLY attacker is a Lv X.
Yet it works. Great.
If a Magnezone deck can manage to get to the mid-late game, then it should be able to have finished setting up, and could take the game.