Oda and Mashima have no proven relation to each other.
My guess is, at the time One Piece was becoming popular, the editors at Shonen Magazine went out to look for a young artist with a similar artstyle to compete with Jump. Even if he was crappy at the time.
666Satan is already finished, at chapter 76. But Fairy Tail could become BIG. I hope Sket Dance gets a little more attention as the time goes on i really like it.
Naruto's Shippuuden is just here to finish off all the mysteries left off b4 shippuuden. Anyways, there are tons of good manga out there, but none has actually sparked me like the big 3. I wish manga would just come out all in a day. Like all 40 volumes, so I don't have to wait a week for a mere 20 pages. I lose interest if there isn't much to read...
If you like manga, then help me out! :]
Manga I enjoy reading/finished reading:
One Piece, Eyeshield 21, Naruto, Bleach, Fairy Tail, Mahou Sensei Negima, Shaman King, D. Gray-Man, Air Gear, Katekyo Hitman Reborn!, Full Metal Alchemist, Psyren, Black Cat, Soul Eater, Satan 666, Death Note, Elfen Lied, Tsubasa: Reservoir Chronicle, Bakuman, King of Thorn