The stones in the plums and the older leaves need to be avoided as they contain cyanide. The young leaves and flowers can be nibbled without harm but should only be offered sparingly, if at all, and the fruit (without the pit) is safe.
For sulcatas of course fruit is unnecessary and in large amounts can be harmful.
One bite is fine, and having a sulcata myself, I know there is little you can do to stop them once they decide they are going to taste something!