Do adults with fever get hallucinations or delusions?

They can. Many febrile illnesses, especially in the elderly can cause delirium, of which delusions and hallucinations, especially visual, are symptoms.
They can. Particularly if the fever is quite high or the person with the fever has a coexisting compromise of the brain like any of the forms of dementia. Remember though, it's not that common for healthy adults with a fever to hallucinate and may suggest a primary infection involving the brain or connective tissue around the brain.