Could a person see and hear imaginary friends as an adult, but not have schizophrenia?

Yes. While common in children, it is rare in adults but not necessarily part of a psychosis. And not all psychotic symptoms are schizophrenia. If there are none of the other diagnostic signs, it's not schizophrenia. If no other disordered thinking, it's not a psychosis. Some writers experience such relationships w/ their characters. Don't worry.