I would've suggested both Mii and Chorus Men above him, but I figured you had already made your mind up to not include them; I doubt I would've been the first to suggest them.
I still would suggest them if you are planning to cut the Trainer (which I think is a good idea). Mii above Chorus Men, but they both seem fairly likely at this point imo.
At this point I'd say getting more than one retro is slim to none. I really would suggest Mii even without Sal's leak though - it seems quite apparent Sakurai is giving a fair amount of attention to Nintendo's last few years, of which Miis played a large part.
You already have a lot of characters though, more than I think we'll get, so you don't really even have to replace Trainer (or even Lip) if you cut them.
I don't believe this Sal guy for a second, I need solid proof before I believe any leak. Sure he got the Wii Fit Trainer, but there have been leaks with right
and wrong info, I remember a particular leak for X/Y that was correct about the new Fairy type as well as plenty of other info, but also had quite a lot wrong as well.
I'm not putting any stock in the Chorus Men, no offense to anyone who actually supported them, but I can't see it with Wii Fit Trainer already in the picture, similar to there usually being a single retro. Miis I feel are in just as bad a spot as Lip, the one thing that made them a unique possibility (moveset customization) is now part of the entire roster, if you can give Kirby ice breath of all things, Miis would just be completely redundant. I don't see either as likely and I'm sticking to my guns.
As for the size, I previously had a 50 slot roster, but when taking into account transformations, it had a total of 55 characters. With transformations gone, it's only logical that my roster would bump up to the actual number of characters. That may have been big before anyway, but people underestimate the roster size.