So knowing Square and how they can be, do you guys think Geno would even be considered a "Mario Series" character if he got in? Do you think he'd have the mushroom emblem like the rest of the Mario cast? For some reason I feel like he'd be separated from the other Mario characters.
Yes. Cause he's not from another series. He's not like Yoshi who had the right timing to promote a current game. He's not like Wario where WarioWare is nothing like the Super Mario series anymore. Wario Land is clearly an offshoot of Super Mario like Yoshi is. WarioWare definitely is not like that. It's not even like Mario Party, the only thing kind of close to it.
Super Mario RPG is still part of the normal Mario series. The only one of the RPG's that really branches out in terms of even artstyle overall is Paper Mario, and that's due to an actual different type of world. Super Mario RPG is in 3D, so while it does have a different artstyle, it's meant to clearly try and emulate the Mario cast quite well, despite having unique monsters who are expies of the actual Mario ones. Chained Kong and its counterparts are literally a Donkey Kong expy. There's a lot of unique ones too, but it doesn't remotely distance itself outright from regular Super Mario. Mario & Luigi originally did distant itself, but for the most part, it did the same thing. So.
It probably helps that Mario in SMRPG and M&L still looks like the current Mario. He doesn't feel like a really different take on it. Paper Mario however has far more changes after being edited from Super Mario RPG 2. They attempted to change small things about his aesthetics. He's no longer happy-go-lucky to the same degree. He clearly takes notes from SMRPG(animations to a degree. How he explains things, though this is present in M&L respectively too). Namely them saying gibberish while telling a story. But hey, SMRPG outright helped create both of these offshoot series. They'd never work without it. It's a pretty noticeable legacy. Paper Mario was able to massively differentiate itself, though Mario & Luigi didn't do so to the same degree. M&L already kept timed hits, but also brought in the same inventory system for equipment specifically. One badge, one armor. Weapons were done a little bit more like Paper Mario, though. The puzzles are also a combination of both games, where transformations are closer to Paper Mario, but the overworld stuff and aiming where to go is clearly from SMRPG.
Getting back ontrack, I don't see why it would need a new symbol. The only one who could make sense is Paper Mario, as he's noted as not the same person as regular Mario, and that doesn't necessarily matter. Also, I mean, it's called Super Mario RPG. Why wouldn't it fit into the Super Mario character series? It's a little easier to justify a lot of games when they have entirely different names too within the franchise. (And let's not even try to get into Donkey Kong Country being a Super Mario spinoff. Ugh. That's always been super misleading. It spunoff from Donkey Kong Arcade specifically. It's the same universe, which is the only reason why the crossovers are constant. Why wouldn't they be? Mario and Donkey Kong know each other. They can easily get together to play Golf, etc. That makes sense. Same with the Wario series, though WarioWare seems to not matter. Etc.)