Thank you for the support! :)
He just gave me a mini heart attack, oh my goodness! I've had him floating in my king's tank to stay warm (I ended up staying home again tonight and my 3g is at school, but at least now he has warm, clean water) and during a water change his cup tipped a little - the little bugger abandoned ship! My king kept flaring at him when I first put Atlas in the tank so I was terrified - and even with that finnage, he is FAST. I couldn't catch him right away, no matter how hard I tried. He had so much room in the 10g he kept darting away from the net. Poseidon went right up to him too, very casually, looked at him, followed him, didn't flare, and just swam away. Thank goodness. This was my face the whole time.
Neither of them flared, chased, nipped, or showed any 'typical' male behavior. Atlas hasn't flared at all yet, actually. Seems pretty relaxed, he'd probably make a great community betta. Within a minute I caught Atlas though and he's quite ticked to be back in his cup. Poseidon has always been quite the aggressive fish so I really thought he would've been rapidly chasing Atlas out of his territory, but didn't show any aggressive behavior at all (Poseidon is quite possibly my favorite betta ever).
On the plus side, Atlas was not lacking in the energy department at all.. so aside from the fin rot, I'd say he's feeling pretty darn good. I didn't realize a halfmoon could swim so fast.