Yes, but it all comes back to the royalty fees thing. Nintendo has no problem including Luigi or Dr. Mario because it won't cost them anything to use the licenses. Mega Man and Zero would cost Nintendo, and they'd be paying money for a pair of characters that would probably play very similarly to Samus or Fox. It would be pointless.
No, Shiek is the cheapest character ever. She has no flaws at all. A newbie that has to have the controls explained to him every time he plays can beat a seasoned veteran with Shiek.
Yoshi comes in at a close second for ultimate cheapness.
falco has a pretty cheap spike move. i don't really see another spike in the game besides his that has the ability to kill so quickly. 50%+ = you are dead automatically if you get spiked by falco.
Yeah, but Samus' ranged attacks are rarely capable of knocking someone out. Only her Charge Beam can kill someone. Well, the Super Missles can sometimes, but only if the person is damn-near dead anyway. Aside from that, Samus has to be up close to kill an opponent.
Plus, she's only got two ranged attacks. The Charge Beam and the Missiles. Her Morph Ball Bombs and Screw Attack are close-range. Link has more ranged attacks than Samus does, so that argument doesn't stack up at all.
A "spike" is a downward aerial attack that sends an opponent flying downwards on contact. If you spike someone while they're trying to get back onto the stage, it can kill them very quickly. Ness used to have the best spike in the original Smash Bros, but his Step Kick sucks in SSBM.