I guess I don't really see why square would loan Tifa to SCVI....
Soul Calibur is a niche fighter and Final Fantasy VIIR is a big game. It's beneath them to loan a FF7 character to this game. They don't really gain anything from it. Nier Automata on the other hand is much closer to SoulCaliburs level, same thing with SamSho. The Witcher wild hunt was 3 years at least past its prime when SC6 came out, so CD Project Red might have had an interest in keeping Geralt going a little bit...
Noctis came out for Tekken when it was less than a year old, plus Tekken 7 is arguably the most popular/reputable fighter this console gen. Its fine to dream for Tifa, but....she's an nearly unattainable dream in SC6.
I could be wrong, but....why the hell would Tifa be picked over Cloud? He's the obvious choice with his sword and this is a weapons based fighter. Save the brawler fighters for either an original Project Soul character or some tekken crossover....
I think your read on this is off in several respects. To begin with, to answer your question regarding SquareEnix's benefit from some a transaction, they stand to gain a lot from it. First off, these deals often include licensing fees, so that's direct renumeration right there--and probably with a per-unit provision: that's essentially completely net revenue. In addition, they gain pretty substantial collateral exposure and marketing signal boosting for their own property in doing so. Nier was a massive commerical success, and yet they still found it worthwhile to license out 2B, even though it was even a little past the heyday sales window for that game.
As for why Tifa rather than cloud: well, there's are some design factors arguing for that: putting aside that Tifa would bring something a little more unique to the formula than just another guy with a massive sword (not exactly something Soulcalibur VI is hurting for..), there's also an element of power levels here: while FFVII:R does kind of reset Cloud to a level of physical ability a little more consistent with Soulcalibur levels of speed and power output, I still think Tifa is more in the wheelhouse.
In addition, there is a certain degree of parity here: Noctis was clearly a character that was better suited to Soulcalibur rather than Tekken (and Azwel is clearly a quasi-butt-hurt reaction from the devs that they didn't get to bite into that character, if you ask me!), and I just wouldn't be surprised if they replicated that inversion thinking into a guest from SC, provided they were of a mind to adapt a character from FFVII:R. There's also the fact that, if we assume that the other five forthcoming characters in Season 2 and hypothetical Season 3 are some combination of Setsuka, Hwang, Rock, Yun-seong, Lizardman, and Li Long, then this entry is going to end with an overall roster significantly skewed towards male characters in terms of outright numbers, so a female guest (and yes, a guest is all but certain if there is a third season) would make the most sense in that respect.
Furthermore, if there is a brawler, it would make the most sense for them to synergize it with the guest: the limitations on who can get in to a final season pass make it pretty much a non-starter to add a new original character brawler in to this game as anybody but a guest. Tekken is another possibility (and one I suspect most of us could get behind), but I just don't think it makes nearly as much sense from a promotional standpoint as Tifa would, especially when you add in the wrinkle that this could be timed (if she is the second season 3 character) to pretty closely coincide with the release of the second volume of FFVII:R, putting her in a pretty nice promotional window between those first two volumes, something that would doubtless appeal to both companies.
