Powdered chemicals should be mixed in a well ventilated area, but generally speaking unless you are mixing raw ingredients most all of the powdered commercial products are coated to protect them from moisture in the package so they give off very little dust unless they have been badly handled.
As far as combustibility, no, nothing that I've ever used for photographic purposes has been flammable or combustible in any way. According to the MSDS sheets that I've seen for photographic chemicals, that should not be an issue. If in doubt look up the MSDS online or at your local library - the librarians can help you find them if you have difficulty tracking them down yourself.
Good luck, and have fun! (oh, and ask questions if you have them...)
- Randy