After 170 something years of ingame experience on my main save, and more carefully recorded observations on my last 2 monsters, I feel safe to state this as a fact. The forte for a heart will always be the tech with the highest level. But being that I usually max all my techs, i wasn't too concerned with that. In the event of more then one tech being maxed, the forte will be the tech that was last maxed.
I had a hunch that the last tech maxed was the forte when i discovered that Pokylyps's forte was Tornado Punch, and remembered that was the tech i last maxed. So when i raised Lisaione, i made sure that the last tech i would max would be Tsunami (aqua orb). Her forte was Tsunami.
Schroder I was raising against the clock, so i didn't care what tech i'd end up as a forte. I cared that i had the best monster possible by the deadline, so i focused on raising Sommersault last, as it benefitted the least from gaining levels of Schroder's techs. In the end, Schroder's forte ended up being Sommersault.
Just noticed this hadn't been mentioned before.

This obviously isn't always the case... I raised an angel who started with a heart that had "star light" on it (already maxed) and then taught her milky way, maxing it out (not only a higher level but also last to be maxed out.) However, her heart forte was still star light. Any idea why?

maybe the forte has something to do with the number of times you use the tech? I once had a tiger which always fought with the tiger charge tech, he then learned the tiger combo whitch I got to level 5 before . . . Mai died. his forte was tiger charge wich was at level 2. point being since he learned tiger combo later in life he used tiger charge more. just a tought. No proof.