Yes, serotonin syndrome is definitely a serious risk. From what I understand, it's typically caused by interactions between SSRIs or MAOIs and substances like methylene blue, rather than methylene blue broadly interacting with many compounds. But I agree - caution is essential when dealing with anything that affects brain chemistry.

More fatal than just taking the SSRIs?

Do they also increase the homicidal ideations?

They can, in unusual instances. This is why doctors quiz you about suicidal ideations whenever you are prescribed a new SSRI.