Don't drink any of them, don't get any in your eyes and if you spill any of them, clean them up.
Dektol might lead to problems for some people in terms of contact allergies - it is never unwise to minimize contact with the skin. I say that despite the fact that I have essentially ignored my own good advice on this subject for 40+ years, without problem.
If you spill fixer on your clothes, it sometimes doesn't wash out well
Dektol, stop bath and fixer aren't going to interact in any potentially harmful way.
Concentrated stop bath is fairly strongly acidic, so a bit extra care is required with it. It is a bad idea to carelessly pour developer into it.