Well, given the way they like to play semantics & leave rules open it perhaps has not been enumerated. I mean, in the exactly one happy Rumple & Milah scene we saw, they seemed to genuinely love each other. Obviously hers was not TL for Rumple. But he carried a torch for her for a long time. Was it TL, or was Belle his first? Can TL go one way? I mean we know Neal considers Emma his TL and she is apparently ambiguous (which is a load of hooey, but whatever,) so maybe it can. Who knows. They haven’t really explained some of the finer points.
I’d say that Rumple loved Milah and Cora but Belle is his only true love. Milah couldn’t stand coward/lame Rumple and Milah couldn’t love him more than she loved herself/power. Belle loved all part of Rumple, even the dark ones.
Charming still loves Snow despite her heart now having a touch of dark.