Forgive me for bringing up old posts, but...
To clear something up, no, Missingno is not an Alakazam, glitch-style. Missingno is a filler glitch for when the game summons a Pokemon that doesn't exist. The game can summon an Alakazam if you follow the same steps of the Missingno glitch and have fulfilled a certain requirement (have the third, fifth, or seventh letter of your name be V, meaning it's not even a happening that occurs most of the time), but unless you see a Kabutops fossil, an Aerodactyl fossil, or a block of static labeled "Missingno," it's not a Missingno.
Not to mention Missingno doesn't have Alakazam's stats or movepool. Yes, I've experimented with TMs/HMs and Missingno. Provided momentary amusement, it did, but Missingno just wasn't that great of a Pokemon itself. I'd much prefer the Alakazam when it comes to stats.
Also, in answer to your question about GSC Missingno, if I recall correctly, they appear as large squares of static.