1. Bowser, Mewtwo, and Dedede were all planned for Smash 64.
Though they had to be scrapped during development due to "circumstances" (most likely hardware issues).
http://www.nintendo.co.jp/n01/n64/software/nus_p_nalj/smash/PostResult2.html
2. The Dedede thing for Melee you described is misconception of an unverified statement by cednym of NeoGAF.
He said that Sakurai stated he:
a) Had backup plans for Dedede if Intelligent Systems did not allow him to include Marth.
b) Could have made one extra non-cloned character in the time it took to make the 6 clones.
People commonly combine these two to mean that Dedede was planned but passed over for the clones. Which is not what cednym said.
3. Just because Sakurai doesn't say someone was considered doesn't automatically mean they weren't.
Aside from that, Diddy Kong was in neither Smash 64 or Melee, was never given any statements by Sakurai on being considered, yet was in Brawl despite being "passed over" twice. Same with Pokémon Trainer Red and his set of Pokémon, Pit, R.O.B.,
Wario, etc. Why should it matter if Ridley or Little Mac got "passed over"?
4. In Melee's time, Ridley and Little Mac would have little reason to be considered;
-Ridley didn't really get any support for Smash until he showed up in Melee's trailer (which is essentially the opening), and even then, the level of support for him didn't escalate to as high as it is until after Brawl was announced (about a year after characters were decided upon other than Sonic). Even then, Metroid wasn't that big of a series in Melee's time, and Ridley was little more than a boss that appeared in 2 out of 3 games. It wasn't until
after Melee that Metroid expanded and Ridley became a much more major character, being tied to Samus' backstory and being
the most recurring character other than Samus herself.
-Little Mac wasn't that big of a request until Brawl's time. ....not really much to say there.