We already have a debuff on the game that makes you unable to crit (consecuences of your dialogue options), what makes you think that you won't get penalized (heavily) from abusing the tadpole? (probably measured on different thresholds, like you said, not going to lock you into an evil ending for having 1 tadpole trait)
We're obviously speculating but we cannot know either way (there's probably a sweet spot where you can minmax the tadpole power and not get too penalized, that much I can assume)
I think the price for abusing the tadpole powers will come in two forms: 1) at some point you will lose companions...either you're forced to kill them or they leave, and 2) you will be locked out of some story paths that would lead to more allies, companions, that sort of stuff.
I don't believe you will necessarily be locked into a bad ending with no way out but you'll probably be locked in a evil aligned ending...though who knows. Maybe some late redemption story arc...though that would most likely not bring back lost party members and allies.
