It's not that Sakurai would decide "hmmmm, yes, we will have exactly one indie character and no more. I have decreed it" at the beginning of the project. It's that negotiating for ONE indie character would likely be exceedingly complicated, so it's more likely that one would be chosen before any other, and that process would make it less likely/feasible to seek another indie character in the same development cycle.
Is there any source for this claim, or is it speculation?
That's an oft-parroted misparaphrasing of a quote from Reggie that basically just said "all the DLC characters will be newcomers". Nothing has been said about all the DLC being from new franchises (though I think that's guaranteed at this point, what with each reveal trailer featuring a "Super Smash Bros X ________" graphic)
If not for the alleged secrecy surrounding the contents of this video, I'd say there's a 5% chance of there being a new character reveal tomorrow. The fact that the video is unscripted only adds to my doubt, because we've seen how much effort and care is put into each character reveal for the pass. We were concerned that the Joker reveal was gonna be the highest-quality reveal trailer we'd get for the FP, with even Sakurai saying not to expect full CG trailers for each character, only for E3 to come around and deliver beautiful, lovingly-rendered trailers for Hero and Banjo. I just don't see Character 4 being revealed as a "oh yeah, here's this person, pre-order now!" type thing.
Honestly, my big concern with tomorrow is whether or not we'll see the Mii Costumes. It seems kinda stupid to NOT showcase the costumes if they're gonna be released the next day, but who knows.