Premiere Elements (video editor) and Photoshop Elements (photo editor) are totally different programs. It's like asking if it's okay to have Word or Reader and PSE on your computer together.
However, it's also okay to have many versions of PSE on your computer. In older versions of windows you shouldn't try to run two versions of PSE at once, but there's no problem at all with having as many versions of the program as are compatible with your operating system.