-Will Plagueis get depressed if I divide his swimming space in half?
Probably not, I've never had mine get depressed when moved into smaller spaces. most actually seem to like it more
-I might buy a female betta. (I've been interested in one) I heard that having a male betta see a female would make them act different. Is that true? If so, what exactly would happen? Would there be any problems?
False. He might blow more nests or become more active seeing a possible mate on the other side. But no real major effects. Before I started my sorority all my males had females in between them and were all perfectly fine. The females were really pushy though and would try to get through any cracks in the dividers that they could.
-If I got a female betta, I could give her 4 gallons and Plagueis 6, right?
You could split it however you want. I always split my 10's so each gets 2.5 gallons (don't do any less than that
-When buying a new one, should I quarantine it? Put it in some sort of dip? I am not sure. (NOTE: I have ParaGuard and Salt.)
Yeah, do a QT of a couple weeks. Don't treat if there's nothing positively wrong with him/her
-Is it okay if the two bettas see each other? What if they get stressed?
Not a problem. If they do get stressed, just add some more craft mesh in front of the divider to distort their view a bit more
-Could Plagueis get stressed if he loses half his space?
he COULD but I highly doubt it
-I'm scared they'll jump over the divider.
that's always a possibility, keep water level 2" lower than the top of the divider and a hood on the tank and you should be okay. OR you could get some type of flat top for it (like craft mesh) so if they do jump, they hit that before they can reach the other side of the divider