Hello dave. A few answers (please note, with the exception of the Sega question, these are all guesses.)
1)"Do you believe that Mario Sunshine and Luigi's Mansion have any relation to each other in terms of gameplay?"
I doubt it. The two games seem very different in premise. One's about a dark haunted house. The other is currently called "Sunshine," for pete's sake! I think Mario Sunshine will be the equivilent to "Super Mario 64 2" and each game will stand alone.
2) "When in the world did Sega and Nintendo join forces?"
They haven't really joined forces, really. Sega has decided to go from being a first party company to being a third party company. They are going to make games for all systems, including Nintendo's GNC and GBA. I, for one, welcome Sonic aboard with open arms!
3)"What are some possible secret characters and stages in SSBM and which ones do you believe will actually be in this seemingly amazing game?"
I've given this a lot of thought. Secret characters I believe will be playable include
Gannondorf, Pit (Kid Icaris), Diddy Kong, Luigi, and Wario, plus others (I hope!). For secret stages, perhaps Gannon's fortress, Angel Land, the island from Diddy Kong Racing, Luigi's Mansion, and the land from Warioland 3. Those are all guesses, of course. We'll all know come December.
Thanks for the questions. Hope that gives you food for thought.
--
I''ve decided to add a signature. Now...what should I type? Hmm...