All the characters in MK:DD have been in previous Mario games, except Toadette. Diddy Kong technically hasn't been in any Mario games (correct me if I'm wrong, which is a moderate possibility, but it's not his debut ). I'll assume the "new" guys you mean include Birdo (originated in Doki Doki Panic, or SMB2), maybe Bowser Jr. (from Super Mario Sunshine and now a Mario game standard). About Luigi's voice, it had been kind of screwy in the past... first it's this, then it's this. Now it's like this! Oh, but now it's THHIIIIIIISSSS. Really annoying. However, it seems to be set in stone as of Luigi's Mansion in 2001. That's the voice in MK:DD. I hope the Super Smash crew know this now and finally fix it.
Now, if you want to see some random Mario characters,, take a look at Mario Golf for the 64. I never approved of a bunch of humans in the Mushroom Kingdom. I... don't even have MG. =P I don't have any sports spin-offs aside from Mario Kart, really. I'll be amazed if this shed any light at all. My head is broken today... I should probably go to sleep already.