Thanks, everyone! He's really pretty chill right now, but last night he found the weak spot in the divider (I'm getting a new five gallon for him soon), and hung out with Phoenix for a while. They didn't seem to really notice each other, but I'm not taking any chances. I've seen Phoenix flare and (try to) chase different fish, so I know he at least has enough aggression in him to do the same to Poseidon, if either one of them would realize they are sharing a tank. I'll have to get a second filter, I think, for Phoenix's half.
I'll definitely grab more pictures, but he has not get flared. We shall see. I'm looking very closely at the caudal fin, and it does not seem to be 180 degrees, so I'm thinking he's just a standard plakat, but maybe he shall be more comfortable and tell me for sure later on.