You can delete photos on your iPad and iPhone from your Camera Roll. They will not be deleted from the PhotoStream when you do this.
Keep in mind though, that PhotoStream is not a permanent shortage location for photos. It is designed to allow photos to be easily and quickly transferred between devices and a computer. Photos will not stay on PhotoStream forever and there is a maximum number of photos that can be stored there.
I suggest you review all your photos and decide which ones you want to save. Then connect your device to your computer, start up your image capture application and copy those photos to your computer. iPhoto works on a Mac and other applications are available on a computer. Once you have your photos on your computer they can be deleted from your Camera Roll.
Create albums on your computer that contain photos you want on your device. Then sync these albums to your device.
There are other cloud based storage services that will allow you to do long term storage of photos and permit your to access them. Even so I would keep a backup of those photos on at least one computer.