I've heard that Ultimate's roster was finalized in... I wanna say 2015, but it might have been 2016. Either year would have been too early to add Rex. Adding a Mii costume, though, takes far less work so they were able to do that. I think the Mii costume bears very little weight right now; it was a sign that Rex probably wasn't going to make it into FP1, but FP2 is a different ballgame. Additionally, Min Min has proven that it doesn't matter much if content relating to a character was added early on - FP2 is a chance to reconsider characters who missed the boat the first time around. At current, I'd say only content that got added shortly before/during FP2 is a sign that we're probably not going to see that character. Since ARMS was back on the table, Rex likely is. It also seems like they're going for characters they wanted to add but couldn't until now, which bodes well for Rex. But take that with a grain of salt, because it's just a guess and the pattern could end up not holding up.