It depends. Sometimes Deku can manage to use 100 percent without getting hurt if he has mastered certain techniques or has received special training. But usually, it comes with a risk of injury.
At first, it could be extremely strained. Deku might be terrified of Bakugou and avoid him at all costs. But as the story progresses, if Bakugou realizes his mistake, he could start trying to make it up to Deku. Deku, on the other hand, might be hesitant to accept Bakugou's apologies at first, but over time, they could start to rebuild a more positive relationship based on mutual respect.
One possible theme could be bullying and its extreme consequences. Bakugou's statement is an extreme form of bullying, and the fanfic might explore how such words can deeply affect Deku's mental state. Another theme could be redemption. Maybe the story will show Bakugou realizing the gravity of his words and trying to make amends. It could also touch on the theme of friendship, as Deku might have to decide whether to forgive Bakugou and rebuild their relationship.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
His friends could come to the rescue. Bakugo, for instance, might use his explosive power to break into Toga's hideout. He's always been competitive with Deku, but he also cares about him deeply.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.