I’m a Chinese-English translator. The technical rules are that only the editor (fluent in both languages) can change other people’s subtitles, since every person’s work is important. With that said, if there are cases where you can edit the grammar mistakes without changing the meaning at all, and people aren’t offended by it, then it seems fine. E.g. Changing “she leaved home” to “she left home” would be fine, but don’t change the actual meaning. I’m new to Viki as well, so not sure if there is a position for “english editor”, if that’s what you’re looking for.