Talles is not feminine. It’s not originally a Hebrew word, and therefore does not need to follow Hebrew grammar, and does not need to be categorized as masculine or feminine. The nekuda on the lamed is not a chirik but a tzeireh or a segol, because in the original Greek that vowel is an epsilon; Ben Yehuda was the one who unilaterally changed it to a chirik.
BTW, Yabia, do you say tehillim or tehilloth?