If you start with a big vocabulary, the child may get overwhelmed and may find it difficult to handle and may even lose interest.
For most beginners, it would be best to start with fewer categories, since it is good to introduce fewer concepts to begin with and then gradually introduce them to more. You can use the Hide option to hide certain icons initially and then Show them later, since this would help introduce concepts one at at time, without deleting the existing words, and also not disrupt their motor patterns. This way, you can start small and without rearranging the vocabulary, you can extend it, making the transition easier for the child.
However, for some children, who are cognitively at a higher level (like children with CP) they might be able to start with almost all categories in the beginning, if they are already aware of the different words and can recognize them.