Would this be ok for the fish if i take all my decor and plants out? I want to do this so i can get a open wide sector and do a complete gravel vacum the whole tank? Would this cause alot of stress? I would add salt after ofcourse.
I would take out some and leave the rest for them if at all possible. Doing it in sections gives the fish a place to go while you're doing the vacuuming.
In the case of Guppies than the salt is fine, otherwise it would be completely unnecessary. Aquarium salt can actually increase the stress as pica suggested because it is actually an irritant for many fish.